Why? Search for a motive in Boston Marathon bombings under way

(CNN) -- With one suspect in the Boston Marathon bombings dead and the other in custody, authorities on Saturday turned their attention to questions about a possible motive in the attack and whether the brothers had help in their alleged plot.

Authorities believe at least some of the answers rest with Dzhokhar Tsarnaev, who was captured Friday night after a shootout just minutes after authorities had indicated that a massive manhunt for the 19-year-old suspect appeared to come up empty.

Federal prosecutors are at the heavily guarded Beth Israel Deaconess Medical Center, where Tsarnaev is in serious condition and being treated for wounds presumably received in a shootout with police.

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 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 massive manhunt that paralyzed Cambridge, Watertown and Boston as authorities searched for a suspect not only believed to be behind the bombings, but who they feared also could unleash more explosives.

Suspect curled up on a blood-stained boat

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 discovered a man curled up in a ball.



"And at that point he became an absolute hero. Instead of being a hero at the moment and yelling at what we now know was the suspect, he did the right thing as law enforcement had urged" and 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 bloodied 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 thermal image photograph, released Saturday by the state police, showed what authorities say was Tsarnaev lying in the middle of the boat.

A gunfight ensued, with more than 20 rounds fired.

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

24 hours of chaos

Shortly after the FBI released photos of the suspects Thursday night, Tsarnaev and his older brother, Tamerlan Tsarnaev, led authorities on a whirlwind manhunt.

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 vehicle, 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. "This is the stuff that in an urban police department, it's almost unheard of."

Tamerlan Tsarnaev died in a shootout. He was wearing explosives and a triggering device when he died, a source briefed on the investigation told CNN.

Over the course of Friday, the Boston area virtually shut down as federal, state and local authorities went door to door searching for Dzhokar Tsarnaev.

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.

Spontaneous parades

The collective mood in the region turned from fear to cheers Friday night after Boston police sent out one tweet: "The hunt is over. The search is done. The terror is over. And justice has won. Suspect in custody."

As local and federal authorities left their positions late Friday night, hundreds of residents swarmed the streets of Watertown and Boston, surrounding the cars and cheering them on.

"Every time a police car passed by, the cheering became louder, and a sense of respect and admiration was felt through the crowd," said Montana Fredrick, who joined a sea of other Northeastern University students in greeting the officers.

"I feel relieved, and I feel like everybody else is relieved," Berklee College of Music student Myles Marcus said. "I feel like I can go back to school now and know that I'm safe."

While many officers remained stoic, members of the Boston SWAT team responded to the crowds: "Thank you. Thank you. It was our pleasure," the team said over a loudspeaker.

Still mourning

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 58 people remained hospitalized late Friday night, including three in critical condition, according to a CNN count.

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.

"It worked, and tonight, our community is once again safe from these two men," the Richard family said.

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

Boston sports teams Saturday were honoring the 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 being 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, for a day-night double-header.

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.

Obama and Republicans stand together

President Barack Obama united with Republicans on Saturday in condemning the violence that took place in Boston and praising the valor that followed.

In his weekly radio address, Obama argued the deadly Boston Marathon bombings, which he described as an "act of terror," will not be remembered for the damage it caused but the "stories of heroism and kindness."

In the Republicans' address, Sen. Tim Scott, like Obama, praised first responders and cautioned those who would consider attacking the United States.

"There is no corner on Earth, no hiding place in America that will keep us from finding you," he said.

Scott said the United States "will do everything in our power to bring justice for the families and the communities impacted. Our freedom is our most precious possession. Any effort to take it away will only strengthen our determination.

'Seek forgiveness'

Tsarnaev's uncle Ruslan Tsarni said their alleged actions were abhorrent.

"You put a shame on our entire family -- the Tsarnaev family -- and you put a shame on the entire Chechen ethnicity," Tsarni said.

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.