Story highlights Tsarnaev has said his brother drove the attack and they had no international ties

Federal investigators return Boylston Street to city of Boston but it remains closed

Tsarnaev is communicating with investigators by nodding, source says

His brother was killed during a police chase early Friday

Boston Marathon bombing suspect Dzhokhar Tsarnaev has told investigators his older brother Tamerlan was the driving force behind last week's attack and that no international terrorist groups were behind them, a U.S. government source said Monday.

Preliminary interviews with Tsarnaev indicate the two brothers fit the classification of self-radicalized jihadists, the source said. Dzhokhar Tsarnaev, wounded and held in a Boston hospital, has said his brother -- who was killed early Friday -- wanted to defend Islam from attack, according to the source.

The government source cautioned that the interviews were preliminary, and that Tsarnaev's account needs to be checked out and followed up on by investigators.

And a federal law enforcement official told CNN that while investigators have seen nothing yet to indicate the suspects were working with anyone else, a lot of work remains before they can say confidently that no others were involved. That official would not comment on any motive or specifics on what Dzhokhar Tsarnaev has communicated to officials.

The 19-year-old has been charged with using and conspiring to use a weapon of mass destruction resulting in death, and one count of malicious destruction of property by means of an explosive device resulting in death. He was heavily sedated and on a ventilator at Beth Israel Deaconess Hospital, but was "alert, mentally competent and lucid" during the brief initial court appearance at his bedside on Monday, U.S. Magistrate Judge Marianne Bowler found.

During the hearing, Tsarnaev communicated mostly by nodding his head, though he once answered "No" when Bowler asked him if he could afford a lawyer, according to a transcript of the proceeding. A public defender was appointed to represent him.

Investigators have been asking Tsarnaev whether there are more bombs, explosives caches or weapons beyond those already found by police, and if anyone else was involved in the attacks, a source with first-hand knowledge of the investigation told CNN. Investigators are going into Tsarnaev's room every few hours to ask questions in the presence of doctors, the source said.

Federal agents at first questioned Tsarnaev without reading him his Miranda rights, under an exception to the rule invoked when authorities believe there is an imminent public safety threat, a Justice Department official said over the weekend. But by the time of the hospital room proceeding, government sources said he had been read his rights, and Bowler reviewed those with him again Monday.

Bowler scheduled a probable cause hearing for May 30.

Tsarnaev had been shot in the head, neck, legs and one hand, according to an FBI affidavit supporting the charges. He had lost a lot of blood and may have hearing loss from two flash-bang devices used to draw him out of the boat, the source said.

-- armed with handguns and explosives -- apparently were planning another attack before the shootout disrupted their efforts. It wasn't clear whether Tsarnaev was wounded during his capture Friday night or in an earlier shootout with police that left his 26-year-old brother dead. Boston Police Commissioner Ed Davis said the brothers-- armed with handguns and explosives -- apparently were planning another attack before the shootout disrupted their efforts.

"I believe that the only reason that someone would have those in their possession was to further attack people and cause more death and destruction," Davis said on CNN's "Starting Point" Monday.

Investigators are also trying to determine whether anyone else was involved in the bombings. But Davis, speaking Sunday to CNN's Don Lemon, said that he was confident that the brothers were "the two major actors in the violence that occurred."

"I told the people of Boston that they can rest easily, that the two people who were committing these vicious attacks are either dead or arrested, and I still believe that," he said.

Meanwhile, after a week of combing the downtown thoroughfare where the bombs went off for evidence, federal authorities handed control of Boylston Street back to the city. But the blocks around the bomb sites remain closed to the public while Boston officials clean up the area and make sure the buildings are safe to occupy.

"This area will be opened up to businesses over the next few hours, and then the people will be back here in a day or so," Davis said. "And they will be walking up and down this street, and the terrorists will understand that they can not keep us down."

A police honor guard, accompanied by a bagpiper, lowered the flag that had flown at the finish line of last week's marathon and presented it to Mayor Thomas Menino to mark the occasion. Shortly afterward, workers in bright yellow suits began hosing down and scrubbing the sidewalks around the second bomb site.

Among the pieces of evidence collected from Boylston Street during the past week was a tree that Dzhokhar Tsarnaev may have leaned against before the bombing, according to a source who receives regular intelligence briefings on the Boston bombings. The source said the tree -- located at the site of the second blast -- was removed along with the surrounding grate, where the explosive device's circuit board was found.

White House: No 'enemy combatant' status

The decision to charge Tsarnaev in civilian court put an end to speculation that he would be charged as an enemy combatant, a designation sometimes used against terrorists. White House spokesman Jay Carney said that Tsarnaev is a naturalized U.S. citizen and cannot be tried by a military commission.

Trying Tsarnaev in civilian courts -- like "hundreds of terrorists" to date -- is "absolutely the right way to go and the appropriate way to go," Carney said. "We have a long history of successfully prosecuting terrorists and bringing them to justice, and the president fully believes that that process will work in this case."

That disappointed Sen. Lindsey Graham, R-South Carolina, who has been calling for Tsarnaev to be handed over to U.S. intelligence as an "enemy combatant."

"There is ample evidence here on the criminal side," Graham said. "A first-year law student could prosecute this case. What I am worried about is, what does this individual know about future attacks or terrorist organizations that may be in our midst? We have the right to gather intelligence."

Graham also said there was also "ample evidence" that the bombings were "inspired by radical ideology."

But while Tamerlan Tsarnaev apparently became increasingly radical in the past three or four years, according to an analysis of his social media accounts and the recollections of family members, there was no evidence Monday that he had any active association with international jihadist groups.

Tamerlan Tsarnaev, 26, died after a shootout with police early Friday. Dzhokhar Tsarnaev was captured that night, after police found him hiding in a boat in the back yard of a house in the Boston suburb of Watertown, Massachusetts.

Older suspect's wife

With one suspect dead and the other hindered in his ability to communicate, investigators are eager to speak to Tamerlan Tsarnaev's wife, Katherine Russell , to see what she might know about incidents leading up to the bombings.

On Monday, her attorney said she learned of her husband's alleged involvement through news accounts.

"She knew nothing about it at any time," Amato DeLuca said in response to questions about whether Russell knew of plans to attack the marathon.

Tsarnaev stayed home and cared for the couple's 2-year-old daughter while his wife worked long hours as a home-care aide, according to DeLuca.

The family is devastated, the attorney said.

"They're very distraught. They're upset. Their lives have been unalterably changed. They're upset because of what happened, the people that were injured, that were killed. It's an awful, terrible thing," he said. "And of course (for) Katy, it's even worse because what she lost -- her husband and the father of her daughter."

Clues about radicalization?

The Tsarnaev family hails from the Russian republic of Chechnya and fled the brutal wars there in the 1990s. The two brothers were born in Kyrgyzstan; Dzhokhar became a U.S. citizen in 2012, while Tamerlan was a legal U.S. resident.

An FBI official said agents interviewed Tamerlan Tsarnaev in 2011 at the request of the Russian government. The FBI said Russia claimed that he was a follower of radical Islam and that he had changed drastically since 2010.

But the Russian government's request was vague, a U.S. official and a law enforcement source said Sunday. The lack of specifics limited how much the FBI was able to investigate Tamerlan, the law enforcement official said.

Photos: Photos: Manhunt for Boston bombers Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Boston bombing suspect Dzhokhar Tsarnaev is detained by officers on Friday, April 19. After a car chase and shootout with police, one suspect in the Boston Marathon bombing, Tamerlan Tsarnaev, was shot and killed by police early Friday, and his brother and second suspect, Dzhokhar Tsarnaev, was taken into custody Friday night. The two men are suspects in the bombings at the Boston Marathon on April 15, that killed three people and wounded at least 170. See all photography related to the Boston bombings. Hide Caption 1 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Special imaging techniques employed by Massachusetts State Police reveal Boston Marathon bombing suspect Dzhokhar Tsarnaev hiding in a boat in a backyard in Watertown on April 19. Hide Caption 2 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Dzhokhar Tsarnaev gets out of the boat he was hiding in outside of a home in Watertown, as seen in a surveillance video still. Hide Caption 3 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – An ambulance carries Boston Marathon bombing suspect Dzhokhar Tsarnaev, 19, from the scene after he was apprehended in Watertown, Massachusetts, on April 19. Hide Caption 4 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Police SWAT teams leave the area after apprehending the suspect in a yard where he was hiding in a dry-docked boat on April 19. Hide Caption 5 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Residents flee from an area where a suspect was hiding on Franklin Street on April 19. Hide Caption 6 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – SWAT team members run toward a police assault on a house as gunfire erupts on April 19. Hide Caption 7 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – People react while watching police respond to reported gunfire on April 19. Hide Caption 8 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – SWAT team members move down residential streets as they perform door-to-door searches in Watertown, Massachusetts, on April 19. Hide Caption 9 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – A U.S. military helicopter lands behind Watertown Mall as law enforcement agencies continue to search for the 19-year-old bombing suspect on Friday. Hide Caption 10 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – SWAT teams prepare to enter a home as they continue the door-to-door search. Hide Caption 11 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – U.S. President Barack Obama meets with members of his national security team in the Situation Room of the White House on April 19 to discuss developments in the Boston bombings investigation. Hide Caption 12 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – SWAT team members line a residential street in Watertown, Massachusetts, as the manhunt continues on Friday. Hide Caption 13 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – A man watches from the window of a home as a SWAT team member keeps watch on Friday, in Watertown, Massachusetts. Hide Caption 14 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Police to continue to the door-to-door search on Francis Street in Watertown, Massachusetts, on Friday. Hide Caption 15 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Law enforcement officers place themselves in an overhead position on Arsenal Street as the search continues on April 19. Hide Caption 16 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Law enforcement officers react to what was initially thought to be a threatening suspect on Arsenal Street on April 19. Hide Caption 17 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – A police SWAT team searches houses on April 19 for the second suspect. Hide Caption 18 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – SWAT teams searches homes along Winsor Avenue in Watertown on April 19. Hide Caption 19 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – A Massachusetts state trooper watches other troopers line up at Watertown Mall as the manhunt for the second suspect continues in Watertown on Friday. Hide Caption 20 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Police continue the ongoing manhunt for the second suspect on Williow Avenue in Watertown on Friday. Hide Caption 21 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – A Watertown police vehicle with bullet holes in its body and a shattered windshield is towed out of the search area on April 19 in Watertown, Massachusetts. Hide Caption 22 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – A Massachusetts State Police officer checks the bag of a cyclist amid heightened security on Friday in Watertown. Hide Caption 23 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Katia Costa looks out her window as police continue the manhunt on Nichols Avenue in Watertown on Friday. Hide Caption 24 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Ruslan Tsarni, uncle of the Boston terror suspects, told CNN affiliate WBZ that Tamerlan "got what he deserved" in an interview outside his home in Montgomery Village, Maryland, on Friday. Hide Caption 25 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – A woman is questioned by Cambridge police and other law enforcement agencies Friday near the home of the second suspect in Cambridge, Massachusetts. A Massachusetts Institute of Technology campus police officer was shot and killed late Thursday night at the school's campus in Cambridge. A short time later, police reported exchanging gunfire with alleged carjackers in nearby Watertown. Hide Caption 26 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – SWAT teams move into position at the intersection of Nichols and Melendy avenues in Watertown, Massachusetts, on Friday. Hide Caption 27 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – SWAT teams gather at the intersection of Nichols and Melendy avenues in Watertown while searching for the remaining suspect on Friday. Hide Caption 28 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Onlookers take pictures while SWAT team members look around on Friday. Hide Caption 29 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Massachusetts Gov. Deval Patrick, left, speaks to the media at a shopping mall on the perimeter of a locked-down area during the search on Friday. Hide Caption 30 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Metro SWAT members hang off the back of a truck during the search on Friday. Hide Caption 31 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – SWAT officers check a door with guns ready on Friday. Hide Caption 32 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Officers patrol Watertown on Friday. Hide Caption 33 of 45 Photos: Photos: Manhunt for Boston bombers Photos: Manhunt for Boston bombers – The aftermath of the shootout that police said involved the two suspects in Watertown early Friday. Hide Caption 34 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Police with guns drawn search for a suspect on Friday in Watertown, Massachusetts. Hide Caption 35 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – A police officer runs with his gun drawn on Friday. Hide Caption 36 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Boston Police gather in the parking lot in front of a Best Buy store near the Watertown Mall on Friday. Hide Caption 37 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Boston Police Commissioner Edward Davis speaks during a media briefing in the parking lot of the Watertown Mall on Friday. Hide Caption 38 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Boston police gather in front of a Best Buy on Friday. Hide Caption 39 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Police stop cars at School and Walnut streets on Friday. Hide Caption 40 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – A Massachusetts state trooper checks a building along Mount Auburn Street as police search neighborhoods in Watertown. Hide Caption 41 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Police search neighborhoods yard by yard on Friday. Hide Caption 42 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Police convene on School and Walnut streets on Friday. Hide Caption 43 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – A police officer talks to a driver at a checkpoint in Watertown, Massachusetts. Hide Caption 44 of 45 Photos: Photos: Manhunt for Boston bombers Manhunt for Boston bombers – Police officers keep a man on the ground in Watertown on Friday. See all photography related to the Boston bombings. Hide Caption 45 of 45

Photos: Photos: Boston celebrates Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A man removes a sign hanging from the Lennox Hotel along Boylston Street after the street reopened to the public for the first time since the Boston Marathon bombings in Boston on Wednesday, April 24. The city is trying to return to normal less than a week after two bombs exploded near the finish line of the Boston Marathon, shocking the nation and leaving the city on edge. See all photography relating to the Boston bombings. Hide Caption 1 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A man washes a bus stop window on Boylston Street on April 24. Hide Caption 2 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Alec Mikels cleans tables at Whiskey's Smokehouse on Boylston Street on Tuesday, April 23 in Boston. Hide Caption 3 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A cleaner power washes Boylston Street near the blast site after the FBI handed the area back to the city of Boston on Monday, April 22, following the week-long investigation. Hide Caption 4 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – The Boston Fire Department Hazardous Materials team cleans the first blast site near the finish line of the Boston Marathon on April 22. Hide Caption 5 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Cleaning material is sprayed on April 22 on the outlined blast seat on the sidewalk of Boylston Street at the site of the marathon bombings. Hide Caption 6 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A member of the Boston Fire Department Hazardous Materials team cleans the first blast site with a pressure washer on April 22. Hide Caption 7 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Brad Marchand of the Boston Bruins embraces one of the first responders from the Boston Marathon attack after the game against the Florida Panthers at the TD Garden on Sunday, April 21, in Boston. Hide Caption 8 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – David Ortiz of the Boston Red Sox speaks to the crowd during a ceremony held in honor of the bombing victims before a baseball game against the Kansas City Royals at Fenway Park in Boston, on Saturday, April 20. Hide Caption 9 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Neil Diamond sings "Sweet Caroline," a song traditionally played at Boston Red Sox home games, during a game against the Kansas City Royals on April 20. Hide Caption 10 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Members of law enforcement react during ceremonies in honor of the Marathon bombing victims before Saturday's game. Hide Caption 11 of 33 Photos: Photos: Boston celebrates Photos: Boston celebrates, seeks return to normal – A woman sheds a tear during pregame ceremonies Saturday. Hide Caption 12 of 33 Photos: Photos: Boston celebrates Photos: Boston celebrates, seeks return to normal – A man holds an American flag at ceremonies before the Saturday game in Boston. Hide Caption 13 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Hundreds of people pour onto Hemingway Street in the Fenway neighborhood to celebrate after the announcement that the second Boston Marathon bombing suspect had been captured on Friday, April 19. Hide Caption 14 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Women cheer police as they exit Franklin Street on Friday, April 19, in Watertown, Massachusetts. Hide Caption 15 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Officers from the Department of Alcohol, Tobacco, Firearms and Explosives relax Friday after the capture in Watertown, Massachusetts. Hide Caption 16 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A woman gives a Boston police officer a hug and other officers are thanked during a celebration in the Boston Common on April 19. Hide Caption 17 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A member of the North Metro SWAT team pumps his fist while leaving the scene near Franklin Street on April 19. Hide Caption 18 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Around 200 people celebrate on Hemingway Street in the Fenway neighborhood after the capture of the second suspect on April 19. Hide Caption 19 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – People wave U.S. flags as police drive down the street on April 19. Hide Caption 20 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A photograph of Martin Richard, one of the victims of the Boston Marathon bombing, was placed on the plexiglass by a fan following the hockey game between the Buffalo Sabres and the New York Rangers on April 19 at the First Niagara Center in Buffalo, New York. Hide Caption 21 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A man waves a U.S. flag in Watertown on April 19. Hide Caption 22 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Local residents cheer to news that police have captured the surviving suspect Watertown on April 19. Hide Caption 23 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – President Barack Obama arrives in the White House briefing room to make a statement late April 19 about the capture of Dzhokhar Tsarnaev. "We've closed an important chapter in this tragedy," he said. Hide Caption 24 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A SWAT team member is followed by reporters and a celebrating crowd on April 19 after the successful operation. Hide Caption 25 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A police officer adjusts his hat while various law enforcement agencies descend on the area around Franklin Street on April 19. Hide Caption 26 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Spectators celebrate as law enforcement officers leave the scene on April 19 near Franklin Street. Hide Caption 27 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – People gather at the scene near Franklin Street in Watertown on April 19. Hide Caption 28 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – A man claps next to a police vehicle in the Watertown neighborhood on April 19. Hide Caption 29 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Onlookers applaud first responders departing the scene at the end of the manhunt on April 19. Hide Caption 30 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Police officers and SWAT team members exult after the successful operation to capture suspect Dzhokhar Tsarnaev on April 19. Hide Caption 31 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Boston SWAT team members are surrounded by spectators and the media on April 19. Hide Caption 32 of 33 Photos: Photos: Boston celebrates Boston celebrates, seeks return to normal – Residents clap after the capture of the second of two suspects wanted in the Boston Marathon bombings on April 19. See all photography relating to the Boston bombings. Hide Caption 33 of 33

Photos: Evidence photos from Boston bombings Photos: Evidence photos from Boston bombings Evidence photos from Boston bombings – A police forensics team examines a boat April 22, 2013, in Watertown, Massachusetts, where Boston Marathon bombing suspect Dzhokhar Tsarnaev was discovered several days earlier and taken into custody. Hide Caption 1 of 15 Photos: Evidence photos from Boston bombings Massachusetts State Police released thermal images of Tsarnaev hiding in the boat on April 19, 2013. They were taken by an infrared device on a helicopter. The first image was taken at 7:19 p.m., less than 20 minutes after a homeowner told police there was a bloodied person in his boat. Hide Caption 2 of 15 Photos: Evidence photos from Boston bombings This image from 7:22 p.m. shows a white heat signature large enough to be a person. Hide Caption 3 of 15 Photos: Evidence photos from Boston bombings A robotic arm tears away the cover on the boat at 7:36 p.m. Hide Caption 4 of 15 Photos: Evidence photos from Boston bombings The heat signature clearly shows the suspect's feet and the rest of his body behind the boat console at 8:01 p.m., minutes before he surrendered. Hide Caption 5 of 15 Photos: Evidence photos from Boston bombings Boston Marathon bomb scene pictures, taken by investigators, show the remains of an explosive device. Hide Caption 6 of 15 Photos: Evidence photos from Boston bombings A federal law enforcement source with firsthand knowledge of the investigation told CNN that a lid to a pressure cooker -- thought to have been used in the bombings -- had been found on a roof of a building near the scene. Hide Caption 7 of 15 Photos: Evidence photos from Boston bombings One bomb was housed in a pressure cooker hidden inside a backpack, the FBI said in a joint intelligence bulletin. Hide Caption 8 of 15 Photos: Evidence photos from Boston bombings The device also had fragments that may have included nails, BBs and ball bearings, the FBI said. Hide Caption 9 of 15 Photos: Evidence photos from Boston bombings The recovered parts include part of a circuit board, which might have been used to detonate a device. Hide Caption 10 of 15 Photos: Evidence photos from Boston bombings A law enforcement official said the bombs were probably detonated by timers. But the FBI said details of the detonating system were unknown. Hide Caption 11 of 15 Photos: Evidence photos from Boston bombings Scraps of at least one pressure cooker, nails and nylon bags were sent to the FBI's national laboratory in Virginia, where technicians will try to reconstruct the devices, the agent leading the investigation said. Hide Caption 12 of 15 Photos: Evidence photos from Boston bombings The U.S. government has warned federal agencies in the past that terrorists could turn pressure cookers into bombs by packing them with explosives and shrapnel and detonating them with blasting caps. Hide Caption 13 of 15 Photos: Evidence photos from Boston bombings The pieces suggest each of the devices was 6 liters (about 1.6 gallons) in volume, a Boston law enforcement source said. Hide Caption 14 of 15 Photos: Evidence photos from Boston bombings "It has the hallmarks of both domestic and international (attacks), and you can see either side of that," former FBI Assistant Director Tom Fuentes told CNN. Hide Caption 15 of 15

Photos: Nation mourns Boston bombing victims Photos: Nation mourns Boston bombing victims Three people were killed when two homemade explosives went off at the Boston Marathon on April 15, 2013, and a campus police officer at the Massachusetts Institute of Technology was fatally shot in the manhunt that followed. From left, the victims were Krystle Campbell, Sean Collier, Lingzi Lu and Martin Richard. Click through the gallery to see how the victims were honored and remembered in the weeks after the terror attack. Hide Caption 1 of 50 Photos: Nation mourns Boston bombing victims People pause at the memorial site in Boston's Copley Square on April 30, 2013. Hide Caption 2 of 50 Photos: Nation mourns Boston bombing victims Running shoes were among the mementos left as a tribute to the bombing victims. Hide Caption 3 of 50 Photos: Nation mourns Boston bombing victims Boston showed its resilience and heart with signs of support for the bombing victims, including this cover from an issue of Boston magazine. Hide Caption 4 of 50 Photos: Nation mourns Boston bombing victims Law enforcement officials enter St. Patrick's Church prior to Collier's funeral in Stoneham, Massachusetts, on April 23, 2013. Hide Caption 5 of 50 Photos: Nation mourns Boston bombing victims President Barack Obama observes a moment of silence in the White House Oval Office on April 22, 2013. Hide Caption 6 of 50 Photos: Nation mourns Boston bombing victims Staff members of the Beth Israel Deaconess Medical Center gather inside a trauma room to observe a moment of silence on April 22, 2013. Hide Caption 7 of 50 Photos: Nation mourns Boston bombing victims A Buddhist sits at a memorial near the marathon finish line during a moment of silence on April 22, 2013. Hide Caption 8 of 50 Photos: Nation mourns Boston bombing victims One week after the bombings, people gather to observe a moment of silence in Copley Square. Hide Caption 9 of 50 Photos: Nation mourns Boston bombing victims Officials line Boylston Street as they observe a moment of silence near the marathon finish line on April 22, 2013. Hide Caption 10 of 50 Photos: Nation mourns Boston bombing victims People take part in the moment of silence near the marathon finish line. Hide Caption 11 of 50 Photos: Nation mourns Boston bombing victims State employees pause for a moment of silence on the steps of the Massachusetts State House. Hide Caption 12 of 50 Photos: Nation mourns Boston bombing victims Pallbearers carry Campbell's casket after a funeral service in Medford, Massachusetts, on April 22, 2013. Hide Caption 13 of 50 Photos: Nation mourns Boston bombing victims Mourners walk out of St. Joseph Catholic Church after Campbell's funeral service. Hide Caption 14 of 50 Photos: Nation mourns Boston bombing victims Nurse practitioner Maureen Quaranto, who treated victims of the bombings, wears her Boston Marathon jacket during Mass on April 21, 2013. Hide Caption 15 of 50 Photos: Nation mourns Boston bombing victims From left, Boston Police Department Superintendents Kevin Buckley and William Evans attend Mass with Police Commissioner Edward Davis on April 21, 2013. Hide Caption 16 of 50 Photos: Nation mourns Boston bombing victims Photos of the deceased are displayed in Boston at the Cathedral of the Holy Cross on April 21, 2013. Hide Caption 17 of 50 Photos: Nation mourns Boston bombing victims Leaders of an interfaith service participate in a vigil near the finish line on April 21, 2013. Hide Caption 18 of 50 Photos: Nation mourns Boston bombing victims A makeshift memorial honors the bombing victims on April 21, 2013. Hide Caption 19 of 50 Photos: Nation mourns Boston bombing victims Nicole Collier Lynch, Collier's sister, hugs a police officer during a vigil in Wilmington, Massachusetts, on April 20, 2013. Hide Caption 20 of 50 Photos: Nation mourns Boston bombing victims People gather at a makeshift memorial in Boston on April 20, 2013. Hide Caption 21 of 50 Photos: Nation mourns Boston bombing victims Huntsville, Alabama, residents hold a prayer vigil on April 19, 2013. Hide Caption 22 of 50 Photos: Nation mourns Boston bombing victims A man rings a bell at the vigil in Huntsville. Hide Caption 23 of 50 Photos: Nation mourns Boston bombing victims Two women embrace during a candlelight vigil in Somerville, Massachusetts, on April 18, 2013. Hide Caption 24 of 50 Photos: Nation mourns Boston bombing victims Obama talks with staff members at Massachusetts General Hospital while visiting injured patients on April 18, 2013. Hide Caption 25 of 50 Photos: Nation mourns Boston bombing victims A man at a Boston restaurant watches Obama speak on television on April 18, 2013. Hide Caption 26 of 50 Photos: Nation mourns Boston bombing victims Mourners fill a Boston cathedral for an interfaith service on April 18, 2013. Hide Caption 27 of 50 Photos: Nation mourns Boston bombing victims Obama, first lady Michelle Obama and Massachusetts Gov. Deval Patrick attend the interfaith prayer service. Hide Caption 28 of 50 Photos: Nation mourns Boston bombing victims Flowers, running shoes and other items are left in memory of bombing victim Lingzi Lu. Hide Caption 29 of 50 Photos: Nation mourns Boston bombing victims Dennis Seidenberg of the Boston Bruins observes a moment of silence before the start of an NHL hockey game in Boston on April 17, 2013. It was the first sporting event held in the city after the bombings. Hide Caption 30 of 50 Photos: Nation mourns Boston bombing victims Danielle Cerroni writes a chalk message on the street near the marathon's finish line on April 17, 2013. Hide Caption 31 of 50 Photos: Nation mourns Boston bombing victims A woman looks at memorials left at the scene of the attack. Hide Caption 32 of 50 Photos: Nation mourns Boston bombing victims A crowd gathers at Boston's Garvey Park during a vigil for bombing victim Martin Richard on April 16, 2013. Hide Caption 33 of 50 Photos: Nation mourns Boston bombing victims A woman uses her hand to keep wind from her candle during an interfaith service in Boston on April 16, 2013. Hide Caption 34 of 50 Photos: Nation mourns Boston bombing victims Mourners gather on the edge of the pond for a candlelight vigil in Boston on April 16, 2013. Hide Caption 35 of 50 Photos: Nation mourns Boston bombing victims Students from the Clifden Academy hold an American flag and candles during a vigil in Dorcester, Massachusetts, on April 16, 2013. Hide Caption 36 of 50 Photos: Nation mourns Boston bombing victims People walk along the barricade at Boylston Street on April 16, 2013. Hide Caption 37 of 50 Photos: Nation mourns Boston bombing victims Mourners in Boston hug one another during a vigil for victims on April 16, 2013. Hide Caption 38 of 50 Photos: Nation mourns Boston bombing victims Five-time Boston Marathon runner Jose Sotolongo, center, reacts during a moment of silence in Miami on April 16, 2013. Hide Caption 39 of 50 Photos: Nation mourns Boston bombing victims The New York Islanders and Florida Panthers stand for a moment of silence before an NHL hocky game in Uniondale, New York, on April 16, 2013. Hide Caption 40 of 50 Photos: Nation mourns Boston bombing victims Members of the Boston Red Sox observe a moment of silence before their Major League Baseball game in Cleveland on April 16, 2013. Hide Caption 41 of 50 Photos: Nation mourns Boston bombing victims From left, Boston Marathon runners Tammy Snyder, Diane Deigmann and Lisa Kresky-Griffin embrace at the barricaded entrance to Boylston Street on April 16, 2013. Hide Caption 42 of 50 Photos: Nation mourns Boston bombing victims Two young girls leave flowers on the steps outside the Boston home of 8-year-old bombing victim Martin Richard on April 16, 2013. Hide Caption 43 of 50 Photos: Nation mourns Boston bombing victims Traders at the New York Stock Exchange observe a moment of silence before the opening bell on April 16, 2013. Hide Caption 44 of 50 Photos: Nation mourns Boston bombing victims The flag above the White House flies at half-staff on April 16, 2013. Hide Caption 45 of 50 Photos: Nation mourns Boston bombing victims The Minnesota Twins stand during the national anthem before a baseball game in Minneapolis on April 15, 2013. Hide Caption 46 of 50 Photos: Nation mourns Boston bombing victims Obama makes a statement about the bombings on April 15, 2013. Hide Caption 47 of 50 Photos: Nation mourns Boston bombing victims Players and fans observe a moment of silence before an NBA game in Oakland, California, on Aprl 15, 2013. Hide Caption 48 of 50 Photos: Nation mourns Boston bombing victims An American flag flies at half-staff at the Capitol building in Washington on April 15, 2013. Hide Caption 49 of 50 Photos: Nation mourns Boston bombing victims Hockey fans in Glendale, Arizona, pause for a moment of silence before a game on April 15, 2013. Hide Caption 50 of 50

In August 2012, soon after returning from a visit to Russia, the elder Tsarnaev brother created a YouTube channel with links to a number of videos. Two videos under a category labeled "Terrorists" were deleted. It's not clear when or by whom.

Tamerlan Tsarnaev attended prayers periodically at the Islamic Society of Boston's mosque in Cambridge, a board member told CNN's Brian Todd. In a statement issued Monday, the society said he twice interrupted sermons -- once in November to express his opposition to celebrating any holiday as un-Islamic, and once in January when he tore into the preacher for citing civil rights leader Martin Luther King.

The second time, the congregation shouted back, "Leave now," the statement said.

"After the sermon and the congregational prayer ended, a few volunteer leaders of the mosque sat down with the older suspect and gave him a clear choice: either he stops interrupting sermons and remains silent or he would not be welcomed," it said. "While he continued to attend some of the congregational prayers after the January incident, he neither interrupted another sermon nor did he cause any other disturbances."

Tamerlan Tsarnaev "began coming intermittently to our congregational prayers on Friday over a year ago and occasionally to our daily prayers," the statement read. "The younger suspect was rarely seen at the center, coming only occasionally for prayer."

Memorials and tributes

One of the victims, Krystle Campbell was memorialized Monday morning in a service at St. Joseph Catholic Church in Medford, Massachusetts. After the service, police officers lined the street in front of the church as other officers wearing dress uniforms saluted as the casket bearing her remains was taken from the church and loaded into a hearse.

Another memorial service was scheduled Monday night at Boston University for Lingzi Lu, a student from China. Lu was a graduate student in mathematics and statistics. Before coming to Boston, she won an academic scholarship to the Beijing Institute of Technology, where she received accolades for her math skills.

On Wednesday, Vice President Joe Biden will attend the memorial service for MIT police officer Sean Collier, who was allegedly killed by the Tsarnaev brothers.

A week after the marathon bombings, 50 people remain hospitalized, including two in critical condition, according to a CNN tally.

At least a dozen survivors have endured amputations.

Patients at Boston Medical Center have received visits from war veterans who have also suffered amputations. The vets, Dr. Jeffrey Kalish said, told patients that their lives aren't over because they've lost limbs.

"We've seen really tremendous success and great attitudes," he said.

Also Monday, Davis -- the Boston police commissioner -- said transit system police officer Richard Donohue, wounded in the firefight with the Tsarnaev brothers, was improving.

"He was in grave condition when he went to the hospital, so we're very optimistic at this point in time, and our prayers are with him and his family," he said.