The suspect, Dzhokhar A. Tsarnaev, 19, was found hiding in a boat just outside the area where the police had been conducting door-to-door searches all day, the Boston police commissioner, Edward Davis, said at a news conference Friday night.

“A man had gone out of his house after being inside the house all day, abiding by our request to stay inside,” Mr. Davis said, referring to the advice officials gave to residents to stay behind locked doors. “He walked outside and saw blood on a boat in the backyard. He then opened the tarp on the top of the boat and he looked in and saw a man covered with blood. He retreated and called us.”

“Over the course of the next hour or so we exchanged gunfire with the suspect, who was inside the boat, and ultimately the hostage rescue team of the F.B.I. made an entry into the boat and removed the suspect, who was still alive,” Mr. Davis said. He said the suspect was in “serious condition” and had apparently been wounded in the gunfight that left his brother dead.

A federal law enforcement official said he would not be read his Miranda rights, because the authorities would be invoking the public safety exception in order to question him extensively about other potential explosive devices or accomplices and to try to gain intelligence.

The Boston Police Department announced on Twitter: “Suspect in custody. Officers sweeping the area,” and Mayor Thomas M. Menino posted: “We got him.”

President Obama praised the law enforcement officials who brought the suspect into custody in a statement from the White House shortly after 10 p.m., saying, “We’ve closed an important chapter in this tragedy.”

The president said that he had directed federal law enforcement officials to continue to investigate, and he urged people not to rush to judgment about the motivations behind the attacks.

The discovery of Mr. Tsarnaev came just over 26 hours after the F.B.I. circulated pictures of him and his brother and called them suspects in Monday’s bombings, which killed three people and wounded more than 170. The case unfolded quickly — and lethally — after that. Law enforcement officials said that within hours of the pictures’ release, the two shot and killed a campus police officer at the Massachusetts Institute of Technology, carjacked a sport utility vehicle, and led police on a chase, tossing several pipe bombs from their vehicle.

Then the men got into a pitched gun battle with the police in Watertown in which more than 200 rounds were fired and a transit police officer was critically wounded. When the shootout ended, one of the suspects, Tamerlan Tsarnaev, 26, a former boxer, had been shot and fatally wounded. He was wearing explosives, several law enforcement officials said. But Dzhokhar Tsarnaev managed to escape — running over his older brother as he sped away, the officials said.

His disappearance, and fears that he could be armed with more explosives, set off one of the most intense manhunts in recent memory. SWAT teams and Humvees rolled through residential streets. Military helicopters hovered overhead. Bomb squads were called to several locations. And Boston, New England’s largest city, was essentially shut down.

Transit service was suspended all day. Classes at Harvard, M.I.T., Boston University and other area colleges were canceled. Amtrak halted service into Boston. The Red Sox game at Fenway Park was postponed, as was a concert at Symphony Hall. Gov. Deval Patrick of Massachusetts urged residents to stay behind locked doors all day — not lifting the request until shortly after 6 p.m., when transit service in the shaken, seemingly deserted region was finally restored.

As the hundreds of police officers fanned out across New England looking for Dzhokhar Tsarnaev, investigators tried to piece together a fuller picture of the two brothers, to determine more about the bombing at the Boston Marathon.
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F.B.I.
The F.B.I. wanted poster for Dzhokhar A. Tsarnaev.
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The older brother, Tamerlan Tsarnaev, was interviewed by the F.B.I. in 2011 when a foreign government asked the bureau to determine if he had extremist ties, according to a senior law enforcement official. The government knew that he was planning to travel there and feared that he might be a risk, the official said.
The official would not say which government made the request, but Tamerlan Tsarnaev’s father said that he traveled to Russia in 2012.
“They had something on him and were concerned about him and him traveling to their region,” the official said. The F.B.I. conducted a review, examining Web sites that he had visited, trying to determine whether he was spending time with extremists and ultimately interviewing him. The F.B.I. concluded that he was not a threat. “We didn’t find anything on him that was derogatory,” the official said.
Now officials are scrutinizing that trip, to see if he might have met with extremists while abroad.
The brothers were born in Kyrgyzstan, an official said, and were of Chechen heritage. Chechnya, a long-disputed Muslim territory in southern Russia, sought independence after the collapse of the Soviet Union and then fought two bloody wars with the authorities in Moscow. Russian assaults on Chechnya were brutal, killing tens of thousands of civilians as terrorist groups from the region staged attacks in central Russia. In recent years, separatist militant groups have gone underground, and surviving leaders have embraced fundamentalist Islam.
The older brother, Tamerlan Tsarnaev, traveled to Russia from the United States last year and returned six months later, on July 17, a law enforcement official said. His father, Anzor Tsarnaev, said his son had mostly stayed with him at his home in Makhachkala, the capital of the Dagestan region, but that the two men had also visited Chechnya.
“We went to Chechnya to visit relatives,” Mr. Tsarnaev said in an interview in Russia.
The trip will come under intense scrutiny to determine whether he met with extremist groups or received training, current and former intelligence and law enforcement officials said. Kevin R. Brock, a former senior F.B.I. and counterterrorism official, said, “It’s a key thread for investigators and the intelligence community to pull on.”
Anzor Tsarnaev, who maintained that his sons were innocent and had been framed, said that during the trip to Chechnya his son had “only communicated with me and his cousins.”
The hunt for the bombing suspects took a violent turn Thursday night, when the two men are believed to have shot and killed a Massachusetts Institute of Technology police officer, Sean Collier, 26, in his patrol car, the Middlesex County district attorney’s office said. After that a man was carjacked nearby by two armed men, who drove off with him in his Mercedes sport utility vehicle.
At one point, the suspects told the man “to get out of the car or they would kill him,” according to a law enforcement official. But they then apparently changed their plans, and forced the man to drive, the official said. At one point, the older brother took the wheel.
“They revealed to him that they were the two who did the marathon bombings,” the official said, adding that the suspects also made some mention to the man of wanting to head to New York. At one point they drove to another vehicle, which the authorities believe was parked and unoccupied. There, the suspects got out and transferred materials, which the authorities believe included explosives and firearms, from the parked car to the sport utility vehicle.
The victim was released, uninjured, at a gas station on Memorial Drive in Cambridge, law enforcement officials said.
After he called the police, they went off in search of his sport utility vehicle, and a frenzied chase began.
The police and the suspects traded gunfire, and “explosive devices were reportedly thrown” from their car, law enforcement officials said. A transit police officer, Richard H. Donohue, was shot in the right leg and critically wounded.
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The Hunt for the Boston Bombing Suspects
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• Eerie Stillness at the Center of a Frenzied Crime Scene (April 20, 2013)
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Officer Donohue had nearly bled to death from his wound when he arrived at the hospital, said a person familiar with his treatment. The hospital’s trauma team gave him a transfusion and cardiopulmonary resuscitation, and got his blood pressure back up, but he was still on a ventilator, the person said.
Finally, the brothers faced off against the police on a Watertown street in what officials and witnesses described as a furious firefight.
A Watertown resident, Andrew Kitzenberg, 29, said he looked out his third-floor window to see two young men of slight build in jackets engaged in “constant gunfire” with police officers. A police vehicle “drove towards the shooters,” he said, and was shot at until it was severely damaged. It rolled out of control, Mr. Kitzenberg said, and crashed into two cars in his driveway.
The gunmen, he said, had a large, unwieldy bomb that he said looked “like a pressure cooker.”
“They lit it, still in the middle of the gunfire, and threw it,” he said. “But it went 20 yards at most.” It exploded, he said, and one man ran toward the gathered police officers. He was tackled, but it was not clear if he was shot, Mr. Kitzenberg said.
The explosions “lit up the whole house,” another resident, Loretta Kehayias, 65, said. “I screamed. I’ve never seen anything like this, never, never, never.”
Meanwhile, Mr. Kitzenberg said, the other man got back into the sport utility vehicle he had been driving, turned it toward officers and “put the pedal to the metal.” The car “went right through the cops, broke right through and continued west.”
He left behind his older brother, Tamerlan Tsarnaev, who had been gravely wounded, and who was brought to Beth Israel Deaconess Medical Center.
Dr. David Schoenfeld, who was catching up on paperwork at his home in Watertown after midnight on Friday, had heard the sirens, and then the gunfire, and the explosions. So he called Beth Israel Deaconess Medical Center, where he works in the emergency room, and told them to prepare for trauma patients for the second time this week.
He said that he arrived at about 1:10 a.m. Fifteen minutes later, an ambulance carrying Tamerlan Tsarnaev pulled up. He was handcuffed, unconscious, and in cardiac arrest, Dr. Schoenfeld said.
As a throng of police officers looked on, Dr. Schoenfeld and a team of other trauma doctors and nurses began to perform CPR.
“There was talk before the patient arrived about whether or not it was a suspect,” Dr. Schoenfeld said. “But ultimately it doesn’t matter who it is, because we’re going to work as hard as we can for any patient who comes through our door and then sort it out after. Because you’re never going to know until the dust settles who it is.”
The trauma team put a breathing tube in the patient’s throat, Dr. Schoenfeld said, then cut open his chest to see if blood or other fluid was collecting around his heart. His handcuffs were removed at some point during the resuscitation attempt, he said, because “when the patient is in cardiac arrest and we’re doing all these procedures, we need to be able to move their arms around.”
The team was unable to resuscitate him, and pronounced him dead at 1:35 a.m. Only as they prepared to turn the body over to the police did Dr. Schoenfeld look closely at the patient’s face and see that he resembled one of the suspects whose pictures had been released by the F.B.I. hours earlier. “We all obviously had some suspicion given the really large police presence,” he said, “but we didn’t have a clear identification from the police.”
Dr. Schoenfeld, whose emergency room treated a number of people injured in the bombings on Monday, said he had not had time to process what he had been through in the early hours of Friday.
“I can’t say what I’ll be feeling as I reflect on this later on,” he said. “But right now I’m more concerned with everybody who’s still out there and still in harm’s way.”
He added, “I worry about everybody in the city, that everyone’s going to be O.K.”

Katharine Q. Seelye reported from Boston, and William K. Rashbaum and Michael Cooper from New York. Reporting was contributed by Richard A. Oppel Jr. and John Eligon from Cambridge, Mass.; Jess Bidgood from Watertown, Mass.; Serge F. Kovaleski and Timothy Rohan from Boston; Ravi Somaiya from New York; Eric Schmitt and Michael S. Schmidt from Washington; Andrew Siddons from Montgomery Village, Md.; Sebnem Arsu from Istanbul; Ellen Barry and Andrew Roth from Moscow; and Andrew E. Kramer from Asbest, Russia.
This article has been revised to reflect the following correction:

Correction: April 19, 2013

An earlier version misspelled the name of a resident who described the police activity in Watertown, Mass. He is Andrew Kitzenberg, not Kitzenburg. An earlier version of this article also misstated where the suspects and police exchanged gunfire. It is Dexter Avenue, not Dexter Street.

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