Story highlights Dzhokar Tsarnaev is "intubated and sedated," an official says

The suspect is in "not yet able to communicate," the governor says

The younger brother suffered an injury to the throat, an official says

57 people remain hospitalized, including three in critical condition

The surviving suspect in the Boston Marathon bombings suffered an injury to his throat and may not be able to talk, a federal official told CNN on Saturday, possibly hindering attempts by authorities to question him about a motive in the attack.

With one suspect dead, authorities believe answers to a motive and whether the brothers had help rest with Dzhokar Tsarnaev, who was captured Friday night just minutes after authorities had indicated that a massive manhunt for the suspect appeared to come up empty.

The official, who was briefed on Tsarnaev's condition, spoke on condition of anonymity.

Tsarnaev was in "serious but stable condition" and "not yet able to communicate yet," Massachusetts Gov. Deval Patrick told reporters during an impromptu briefing on Saturday.

Federal prosecutors are at the heavily guarded Beth Israel Deaconess Medical Center, where Tsarnaev is being treated for wounds.

Authorities have not publicly detailed the injuries sustained by the teen, but an official who has been briefed said Tsarnaev has been "intubated and sedated." The official also spoke on condition of anonymity.

"I, and I think all of the law enforcement professionals, are hoping for a host of reasons that the suspect survives, because we have a million questions, and those questions need to be answered. There are parts of the investigation, in terms of information and evidence, that still needs to be run to ground," Patrick said.

Authorities have not said whether they have questioned Tsarnaev, nor have they publicly said what charges will be filed against the teen. But a Justice Department official, speaking on condition of anonymity, told CNN the teen will face federal terrorism charges and possibly state murder charges.

The government has invoked the public safety exception, a designation that allows investigators to question the teen without reading him his Miranda rights and without a lawyer present, another Justice Department official, also speaking on condition of anonymity, told CNN.

Tsarnaev, 19, and his older brother, 26-year-old Tamerlan Tsarnaev, are accused of setting off two bombs near the finish line of the Boston Marathon on Monday, killing three people and leaving more than 170 wounded.

The arrest of Tsarnaev brought to an end a manhunt that paralyzed Cambridge, Watertown and Boston as authorities searched door to door for a suspect not only believed to be behind the bombings, but who they feared also could unleash more explosives.

Acting alone?

So far, evidence suggests that the two brothers acted alone in the bombings and subsequent shootout, Watertown Police Chief Edward Deveau said.

"From what I know right now, these two acted together and alone," Deveau told CNN on Saturday. "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."

Even so, questions remain.

"Why did these young men who grew up and studied here as part of our communities and our country resort to such violence? How did they plan and carry out these attacks? And did they receive any help? The families of those killed so senselessly deserve answers," President Barack Obama said Friday in a televised address.

24 hours of chaos

Shortly after the FBI released photos of the suspects Thursday night, Tsarnaev and his older brother led authorities on a wild car chase and shootout.

Officials say the brothers, for no obvious reason, killed Massachusetts Institute of Technology police officer Sean Collier while he was sitting in his car. The Tsarnaevs then hijacked a Mercedes, telling the driver they were the marathon bombers, and hurled explosives at the pursuing officers, authorities said.

"There was an exchange of over 200 rounds of gunfire, there were improvised explosive devices, and handmade hand grenades thrown at the officers at the scene," Boston Police Commissioner Ed Davis said.

Tamerlan Tsarnaev ran out of ammunition during the shootout and was tackled by officers. That's when the younger Tsarnaev drove the Mercedes toward the officers and his brother.

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: 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

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

"They dive out of the way, and he (the younger brother) drives over his brother and drags him a short distance down the street," Deveau said.

Tamerlan Tsarnaev was later pronounced dead at the hospital. He was wearing explosives and a triggering device when he died, a source briefed on the investigation told CNN.

The FBI, while executing a search warrant at a residence believed to have been affiliated with Tsarnaev, took three people into custody for questioning Friday. It was unclear Saturday what information the FBI was seeking and whether the three were released.

'Pool of blood'

The dramatic end to the manhunt came when a Watertown man, cooped up in his house all day because of a "shelter in place" order, finally stepped outside when the order was lifted.

David Henneberry soon noticed the tarp covering the boat in his backyard was flapping in the wind and a retention strap was cut. He also noticed a small amount of blood on the tarp.

"He basically stuck his head under the tarp, noticed a pool of blood," Henneberry's stepson Robert Duffy told CNN

Henneberry called 911, Duffy said.

Authorities arrived and evacuated Duffy's stepfather. Using a bullhorn, they called out to the suspect: "Come out with your hands up."

The man refused.

"We used a robot to pull the tarp off the boat," David Procopio of the Massachusetts State Police said. "We were also watching him with a thermal imaging camera in our helicopter. He was weakened by blood loss -- injured last night, most likely."

A gunfight ensued, with more than 20 rounds fired.

Authorities eventually rushed the boat and took the teen into custody.

Cheers and mourning

As word of the arrest spread, hundreds of residents swarmed the streets of Watertown and Boston, surrounding police cars and cheering them on.

But the celebrations were tempered by the deaths of four people this week, all allegedly by the hands of the Tsarnaev brothers.

Three spectators were killed in the marathon bombings, and Collier, the MIT police officer, was fatally shot early Friday. At least 57 people remained hospitalized Saturday afternoon, including three in critical condition, according to a CNN count.

On Saturday night as Collier's body was driven from the morgue to a funeral home, Boston-area police officers and firefighters lined the route to pay respect to their slain colleague.

The family of 8-year-old Martin Richard , who was killed in the bombings, issued a statement thanking the authorities and members of the public who helped track down the two suspects.

"None of this will bring our beloved Martin back, or reverse the injuries these men inflicted on our family and nearly two hundred others," the Richard family said. "We continue to pray for healing and for comfort on the long road that lies ahead for every victim and their loved ones."

'Boston Stands as One'

Boston sports teams Saturday honored victims of the attacks.

The Boston Red Sox planned a special pre-game ceremony at their Saturday game against the Kansas City Royals, which was played amid heightened security at Fenway Park. Their Friday night game against the Royals was postponed because of the city lockdown and will be played Sunday, the team said.

The Boston Bruins hockey game against the Pittsburgh Penguins, originally scheduled for Friday night, was rescheduled for Saturday afternoon.

The Bruins and Penguins, along with the Red Sox, all plan to auction their Saturday jerseys to support the bombing victims.

Limited-edition T-shirts reading "Boston Stands as One" are being sold by the Boston Celtics to support the victims. Players planned to wear some of the shirts while warming up for Saturday's game, the team said.

'Seek forgiveness'

The Tsarnaevs' uncle Ruslan Tsarni said their alleged actions were abhorrent.

Tsarni promised Saturday to help his nephew seek forgiveness from the bombing victims and advised him to tell police everything he knows.

The brothers come from a family originally from the Russian republic of Chechnya and fled the brutal wars there in the 1990s. It's unknown how their Chechen roots may have influenced their alleged actions.

Tsarni said he believes Tamerlan Tsarnaev influenced his younger brother.

FBI agents interviewed the elder Tsarnaev in 2011 at the request of a foreign government that suspected he had ties to extremist groups, the FBI said. It declined to name the government, but a senior U.S. official told CNN on Saturday that it was Russia that made the request of the United States.

The request was based on information that Tamerlan Tsarnaev was a follower of radical Islam, the FBI said, adding it found no evidence of terrorism activity.

"I think unless we see some horrible dropping of the ball, I don't think this is an intelligence failure," said former CIA operative Robert Baer. "In retrospect, it might look like one, but I don't think it is."

The suspects' father, who lives in the Russian republic of Dagestan, told CNN on Saturday that he believes his sons were "never, ever" involved in the Boston attacks. He also said he plans to go to the United States, though he didn't say when.