Story highlights 55 people remain hospitalized from the marathon bombings

Doctors are ''cautiously optimistic'' a wounded transit officer will recover

The surviving suspect was shot in the side of the neck, a source tells CNN

Based on evidence, the brothers were going to attack others, an official says

Federal prosecutors were preparing charges Sunday against the surviving suspect in the Boston Marathon bombings even as authorities said they believed he and his brother were allegedly preparing to carry out more attacks when their plans were disrupted.

Authorities have not said publicly what charges will be filed against Dzhokhar Tsarnaev, but a Justice Department official, who has been briefed on the case, told CNN he will face federal terrorism charges and possibly state murder charges.

Tsarnaev, 19, remains in serious but stable condition with a gunshot wound to the side of the neck, a federal law enforcement source with knowledge of the investigation told CNN on Sunday.

It is unclear whether Tsarnaev was wounded during his capture or an earlier shootout with police that left his older brother -- the other man wanted in the bombings -- dead, said the source, who spoke on condition of anonymity.

"He's not in a condition to be interrogated at this time," Boston Police Commissioner Ed Davis told reporters at a news conference on Sunday afternoon.

However, another senior federal official who has been briefed on the investigation told CNN's Fran Townsend that Tsarnaev has communicated in writing with officials several times.

Even as prosecutors worked to put together a case against Tsarnaev, Davis said he believed the brothers were planning another attack before a shootout with police disrupted their plans.

"We have reason to believe, based upon the evidence that was found at the scene -- the explosions, the explosive ordnance that was unexploded and the fire power -- that they were going to attack other individuals," Davis said Sunday on CBS News' "Face the Nation."

He did not say whether investigators had identified a specific target.

Authorities believe the brothers bought bomb components locally, but their guns came from elsewhere, another federal law enforcement official with knowledge of the investigation told CNN. The official, who was not authorized to publicly discuss the case, said authorities are trying to trace the guns.

Tsarnaev was captured Friday night, days after he and his brother allegedly planted two bombs near the finish line of the Boston Marathon. The blasts killed three people and wounded more than 170 others.

Authorities virtually shut down Boston and some of its suburbs as part of a manhunt after authorities say the brothers went on a rampage late Thursday and early Friday. The brothers allegedly hurled explosives at police after killing Massachusetts Institute of Technology police Officer Sean Collier and hijacking a car.

What's next for the suspect?

There are questions about whether Tsarnaev will ever be able to talk given the nature of his wound, Sen. Dan Coats, R-Indiana, said Sunday on ABC's "This Week."

"It doesn't mean he can't communicate, but right now I think he's in a condition where we can't get any information from him at all," said Coats, a member of the Senate Armed Services Committee.

The government has invoked the public safety exception in the case, a designation that allows investigators to question Tsarnaev without reading him his Miranda rights and without a lawyer present, said another Justice Department official, also speaking on condition of anonymity for the same reason.

Secretary of Defense Chuck Hagel told reporters on a flight to Israel that he has not seen any intelligence that linked the brothers to any terrorist organization, but it was still early in the investigation.

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: 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: The day Boston became a ghost town Photos: The day Boston became a ghost town Summer Street in downtown Boston is empty as authorities hunt for the Boston Marathon bombing suspect on April 19, 2013. Much of the Boston area was closed or on lockdown during the investigation, and residents were asked to stay inside. Hide Caption 1 of 21 Photos: The day Boston became a ghost town A man crosses a vacant Beacon Street, across from Boston Common. Hide Caption 2 of 21 Photos: The day Boston became a ghost town Federal Street, in the Financial District Hide Caption 3 of 21 Photos: The day Boston became a ghost town The Boston Red Sox postponed a baseball game because of the manhunt. Hide Caption 4 of 21 Photos: The day Boston became a ghost town Moody Street was deserted in nearby Waltham, Massachusetts. Hide Caption 5 of 21 Photos: The day Boston became a ghost town Congress Street in Boston Hide Caption 6 of 21 Photos: The day Boston became a ghost town Beacon Street, with a view toward the Massachusetts State House Hide Caption 7 of 21 Photos: The day Boston became a ghost town The area near Boston's Downtown Crossing would usually be filled with lunchtime crowds. Hide Caption 8 of 21 Photos: The day Boston became a ghost town The Boston Public Garden Hide Caption 9 of 21 Photos: The day Boston became a ghost town Congress Street at Milk Street Hide Caption 10 of 21 Photos: The day Boston became a ghost town The Boston Public Garden Hide Caption 11 of 21 Photos: The day Boston became a ghost town The area around the North Station next to the TD Garden Hide Caption 12 of 21 Photos: The day Boston became a ghost town The Harvard Bridge, known locally as the Massachusetts Avenue Bridge Hide Caption 13 of 21 Photos: The day Boston became a ghost town A lone bicyclist makes his way across the Longfellow Bridge. Hide Caption 14 of 21 Photos: The day Boston became a ghost town Watertown Square Hide Caption 15 of 21 Photos: The day Boston became a ghost town Friend Street near North Station Hide Caption 16 of 21 Photos: The day Boston became a ghost town A flower sits near the site where a police officer was killed in nearby Cambridge, Massachusetts. Hide Caption 17 of 21 Photos: The day Boston became a ghost town Ceasar McDowell, an urban studies professor at the Massachusetts Institute of Technology, says he went for a walk only to find everything closed except for a CVS drugstore and a Dunkin' Donuts. "I don't know how to describe it," said McDowell, a CNN iReporter. "(I)t was eerie that quietness." Hide Caption 18 of 21 Photos: The day Boston became a ghost town Massachusetts Avenue in Cambridge Hide Caption 19 of 21 Photos: The day Boston became a ghost town The area around North Station Hide Caption 20 of 21 Photos: The day Boston became a ghost town The usually bustling Boston University campus Hide Caption 21 of 21

Photos: Boston bomber Dzhokhar Tsarnaev Photos: Boston bomber Dzhokhar Tsarnaev Dzhokhar Tsarnaev was arrested on April 19, 2013, after a massive manhunt. An overnight shootout with police killed the other suspect -- Tsarnaev's 26-year-old brother, Tamerlan. A jury condemned Tsarnaev to death on Friday, May 15, for his role in killing four people and wounding hundreds more. Hide Caption 1 of 11 Photos: Boston bomber Dzhokhar Tsarnaev On April 18, 2013, the FBI released photos and videos of two suspects and asked the public to help identify them. Hide Caption 2 of 11 Photos: Boston bomber Dzhokhar Tsarnaev Dzhokhar Tsarnaev was found on April 19, 2013, in a boat that was dry-docked in the backyard of a Watertown home. He was covered in blood from bullet wounds. Hide Caption 3 of 11 Photos: Boston bomber Dzhokhar Tsarnaev The August 2013 cover of Rolling Stone featured Tsarnaev and sparked a backlash against the magazine. Hide Caption 4 of 11 Photos: Boston bomber Dzhokhar Tsarnaev Tsarnaev stands in court, flanked by his lawyers, in this sketch from July 2013. Hide Caption 5 of 11 Photos: Boston bomber Dzhokhar Tsarnaev An image posted to the social sharing website Reddit purportedly shows Dzhokhar Tsarnaev being detained by law enforcement officers. Hide Caption 6 of 11 Photos: Boston bomber Dzhokhar Tsarnaev Tsarnaev was seen on this convenience store surveillance video that was released by the Boston Police Department. Hide Caption 7 of 11 Photos: Boston bomber Dzhokhar Tsarnaev A still of the suspects from footage released by the FBI after the bombing. Hide Caption 8 of 11 Photos: Boston bomber Dzhokhar Tsarnaev Additional photos and video were released by the FBI. Hide Caption 9 of 11 Photos: Boston bomber Dzhokhar Tsarnaev A picture of Tsarnaev from his apparent profile on VKontakte, a Russian social network similar to Facebook. Hide Caption 10 of 11 Photos: Boston bomber Dzhokhar Tsarnaev The Boston Police Department also released this undated photograph of Tsarnaev. Hide Caption 11 of 11

But Davis told CNN 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 Sunday.

The sentiment was echoed by Watertown Police Chief Edward Deveau.

"From what I know right now, these two acted together and alone," he said. "I think we have to be ever vigilant, and we're learning as we go along, but as far as this little cell -- this little group -- I think we got our guys."

Hints of a radical?

While investigators piece together the actions of the brothers in the months and days before the marathon bombings, there appear to be hints that the elder Tsarnaev was becoming radical.

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, authorities said.

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

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

Tamerlan Tsarnaev apparently became increasingly radical in the last three or four years, according to an analysis of his social media accounts and the accounts of family members. But so far, there is no evidence of active association with international jihadist groups.

In August 2012, soon after returning from his visit to Russia, the elder Tsarnaev 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.

A CNN analysis of the YouTube channel has established that one deleted video featured a militant named Abu Dujana, whose real name was Gadzhimurad Dolgatov. CNN has located a video clip of the footage in question.

Russian security services killed Dolgatov in December during an assault on an apartment in Makhachkala, the capital of the Russian Caucasus republic of Dagestan. Dolgatov led a small group in Dagestan that had links to the main Islamist militant group in the region, Imarat Kavkaz.

Separately, a U.S. intelligence source told CNN that investigators are looking into whether Tsarnaev had any connections with the group, known in English as Caucasus Emirates. The source says Tsarnaev had several computer links to the group in his social media activities, and investigators are looking into the possibility that he received "operational plans" from this group.

Rebels who call themselves Mujahideen of the Caucasus Emirate Province of Dagestan issued a statement Sunday that appeared to distance the group from the Boston bombings, saying they are not fighting the United States.

"We are at war with Russia," it said. The statement also said that children are never targets of the group.

'He was just relaxed'

As an army of officers hunted for the suspects in Monday's marathon bombings, Dzhokhar Tsarnaev acted like any other college sophomore.

He was on the campus of University of Massachusetts Dartmouth every day after the attack until late Thursday, a university official told CNN. Tsarnaev attended classes and went to the gym and dorm parties while much of Boston was at a tense standstill.

On Tuesday, he and a fellow student were at the campus gym when the bombings came up in discussion.

Zach Bettencourt said he expressed his shock about the attack, telling Tsarnaev that it was the type of thing you hear about in Iraq or Afghanistan, not Boston.

He said Tsarnaev responded: "Yeah, tragedies happen man. Like these things happen around the world. It's crazy."

A student at the school told The Boston Globe she saw Tsarnaev Wednesday night at a party that was attended by some of his friends from intramural soccer.

"He was just relaxed," she said, asking the paper not to print her name.

At the dorm where Tsarnaev lived, students joked Thursday as they viewed the FBI photos of the bombing suspects on television, a senior who lived in that dorm told The Boston Globe.

"We made a joke like, that could be Dzhokhar," Pamala Rolon said. "But then we thought it just couldn't be him. Dzhokhar? Never."

The campus, which was closed during the search for the bombing suspects, reopened Sunday morning.

Moment of silence

Boston, meanwhile, is trying to return to semblance of normalcy with some streets and business reopening.

Federal investigators on Sunday were lifting crime scene tape and barriers erected during the investigation into the bombings near the marathon's finish line on Boylston Street.

People throughout Massachusetts are being urged to observe a moment of silence Monday at 2:50 p.m., exactly one week after the Boston Marathon bombings.

Of those injured in the marathon bombings, 55 remain hospitalized, including three in critical condition, according to a CNN count.

Doctors, meanwhile, said Sunday that they are "cautiously optimistic" that a Massachusetts transit officer wounded in a shootout with the alleged Boston Marathon bombers will recover.

Richard H. Donohue, 33, remains in critical condition at Mount Auburn Hospital in Cambridge, said Dr. Russell J. Nauta, a surgeon who operated on the officer.

There was life before the bombings, and then life after.

"We are all scattered in the pain and horror of this week's violence," Boston's archbishop, Cardinal Sean O'Malley, told parishioners during Sunday services at the Cathedral of the Holy Cross.

"Some of those here were among those injured. But everyone has been affected."

He voiced what so many have been thinking: Why would anyone do this? What were the bombers thinking?

It has been "very difficult to understand what was going on in their heads," he said.