The suspect, 19-year-old Dzhokhar Tsarnaev, was taken into police custody after hiding in a boat in a residential backyard in suburban Watertown, the epicentre of a massive manhunt all day.
His capture came after a stunning chain of events that transfixed a nation at the end of a dreadful week. On Monday, a pair of bloody bombings at the famed marathon killed three people, including an eight-year-old boy, and wounded more than 170 others, some of them grievously.
U.S. President Barack Obama praised Americans in remarks at the White House after Tsarnaev's capture.
"All in all, this has been a tough week, but we’ve seen the character of our country once more," Obama said.
"And as president, I’m confident that we have the courage and the resilience and the spirit to overcome these challenges and to go forward."
He added "there are still many unanswered questions" about the bombings, and he's directed the FBI and other agencies to get those answers.
"We will determine what happened," he said while also cautioning Americans not to "rush to judgment."
Tips about the identities of Tsarnaev and his 26-year-old brother, Tamerlan, began pouring into the FBI soon after the agency released their images publicly on Thursday afternoon, setting off a rapid-fire sequence of developments.
The FBI got a huge helping hand from Jeff Bauman, the subject of the horrifying photo taken immediately after the bombing that showed him with both legs blown off below the knee.
Bauman, 27, provided a highly detailed description of Tamerlan Tsarnaev — even sketching him for police from his hospital bed on Wednesday night — that allowed investigators to zero in on a culprit as they pored over a department store surveillance tape of the crowds gathered at the race's finish line.
Bauman saw the man drop a knapsack at his feet shortly before the blasts.
"He woke up under so much drugs, asked for a paper and pen and wrote, 'bag, saw the guy, looked right at me,'" Bauman's brother, Chris, told the Bloomberg news organization earlier this week.
Within hours of the release of their images, authorities say the Tsarnaev brothers had gunned down Sean Collier, 26, a police officer at the Massachusetts Institute of Technology, as he sat in his cruiser at 10:20 p.m. Collier had not drawn his weapon.
Police say the brothers then hijacked a Mercedes SUV, holding the driver captive for a half-hour while they used his cash card to withdraw $800 at an ATM. The man, who told police the brothers had confessed their crimes to him, was released unharmed at a gas station in Cambridge, across the Charles River from Boston.
The suspects then sped west in the SUV car toward Watertown, with police in hot pursuit as explosive devices were tossed at them out the window. The brothers soon engaged in a gun battle with police that left Tamerlan dead and Dzhokhar at large after he drove the vehicle over his wounded brother.
Authorities reportedly attempted to track him by a trail of blood after he abandoned the vehicle and fled on foot.
Among the explosives hurled during the firefight was another pressure-cooker bomb of the type used in the marathon blasts, CBS News reported. The pressure cooker's lid apparently came loose as it was thrown, preventing its full detonation.
There was a major explosion during the gun battle, however, evident on audio and video tape of the mayhem. The New York Times also reported that Tamerlan Tsarnaev may have had a homemade bomb strapped to his body when he went down in a hail of police gunfire.
As dawn broke, heavily armed police swarmed Boston and its suburbs, placing the metropolitan area on lockdown as they searched for Tsarnaev. By late afternoon, they conceded their search had thus far been fruitless.
But authorities had barely wrapped up a news briefing to lift a "stay indoors" order when news broke that they had Tsarnaev cornered in a boat in winter storage in a residential backyard.
They were reportedly tipped off by the owner of the house, who noticed the tarp covering his pleasure boat had been slashed when he went outside for a cigarette. When he peeked inside, he saw a pool of blood and a body and called 9-11.
After a brief standoff and the arrival of police negotiators, authorities announced that Tsarnaev had given himself up and was being transported to hospital. He was in serious condition.
"We got him," one officer said as neighbours erupted into cheers and applause and high-fived the hundreds of law enforcement officials on the scene.
Details about the two brothers, who immigrated to the U.S. about a decade ago, emerged at a fast and furious pace all day as family members and associates spoke openly to news media while social media disclosed a treasure trove of personal details.
An array of relatives, including their Toronto aunt, came forward to either fiercely defend the brothers or bitterly condemn them.
"I need evidence," Maret Tsarnaev told an impromptu news conference in west-end Toronto. "Show me evidence. I don't trust FBI, I don't trust any agency."
An uncle, Ruslan Tsarni, was asked outside his Maryland home what would have provoked his nephews.
"Being losers," he replied angrily, adding that his brother had simply "fixed cars" to put food on the table for his family while living in America and bore no responsibility.
"Dzhokhar, if you are alive, turn yourself in, ask forgiveness from these people," Tsarni said to news cameras congregated on his driveway. "You brought shame on our family, the entire Chechnya people. You put this shame on our entire ethnicity."
The brothers' father — Anzor Tsarnaev, now in Russia — also urged his surviving son to give himself up.
He told ABC News he spoke to his sons earlier this week, after the blasts at the famed marathon.
"We talked about the bombing. I was worried about them," Anzor Tsarnaev said from Russia. He said his sons reassured him, saying: "Everything is good, Daddy. Everything is very good."
Their New Jersey sister, meantime, said she was shocked.
"They were great people. I never would have expected it," Alina Tsarnaev told the New Jersey Star-Ledger outside her home. "They are smart — I don't know what's gotten into them."
The brothers were from Dagestan, which borders Chechnya in southern Russia, and initial reports suggest they were adrift after their parents returned to Russia.
Tamerlan, a boxer, was the subject of a photo essay entitled "Will Box For Passport" taken before he competed at the National Golden Gloves competition in Salt Lake City in 2010.
"I don't have a single American friend, I don't understand them," he said, according to the caption on one of the photos.
Chechens are Russians by citizenship but not by nationality. Famously tough fighters, they complain that Russians frequently discriminate against them.
Chechnya has been the scene of two wars between Russian forces and separatists since 1994. The conflicts kicked off an Islamic insurgency in the region.
On his page on Vkontatke — the Russian-language equivalent of Facebook — Dzhokar Tsarnaev, who became a U.S. citizen last Sept. 11, tells a joke.
"A car is driving down a street. In it are a Chechen, an Ingush and a Dagestani. Question: Who's behind the wheel? Answer: a cop."
In the U.S. capital on Friday, federal counterterrorism officials were trying to determine if the brothers were part of a larger operation with accomplices still at large. They're also investigating whether they had any connections to foreign or domestic terrorist organizations.
By most accounts, Dzhokar Tsarnaev, an avid wrestler, was more popular than his older brother and had a network of friends. Some of them expressed shock at his involvement in the bombings, describing him as kind and intelligent.
Tsarnaev was a student at the Dartmouth campus at the University of Massachusetts and had a dorm room there. Two students told CBS News that they'd seen Tsarnaev on campus this week, after the bombings.
The school was evacuated on Friday as police descended upon the campus to scour his room.
By noon on Friday, a sea of law enforcement officers had also surrounded the Cambridge home where the brothers grew up and where Tamerlan was thought to have still resided. Police said they feared it could be booby-trapped with explosives and conducted a controlled explosion.
Subways and buses were shut down while Amtrak service to Boston was halted. Harvard University, Boston University, the Massachusetts Institute of Technology and Emerson University were all closed and students were told to stay inside.
Major-league baseball and hockey games were cancelled.
Note to readers: Fixes age of bomb victim