WATERTOWN, Mass.— Police said they had taken the second suspect in the Boston Marathon bombings into custody here Friday night, after a day of intense searching that shut down daily life across a large swath of greater Boston.

Shortly after 8 p.m., police surrounded a boat stored behind a home in East Watertown, a short distance away from where Dzhokhar Tsarnaev, 19, had been last seen. Authorities said they believed Tsarnaev was inside the boat, which had been covered in a tarp. He was thought to be wounded but alive: television crews reported that they could hear police calling his name, attempting to induce his surrender.

The other suspect in the bombings, Tsarnaev’s 26-year-old brother, Tamerlan, was killed early Friday morning after a shootout with police in another section of Watertown.

Also Friday, a report from Russian television cited the men’s mother as saying her older son had previously been interviewed by the FBI because of his interest in radical Islamic teachings. The FBI confirmed that agents in Boston had interviewed the elder Tsarnaev in 2011, on behalf of an unspecified foreign government that suspected he had ties to a terrorist organization. But the FBI found nothing warranting further investigation.

The standoff with the younger Tsarnaev began just minutes after a press conference in which authorities had conceded that a daylong search for Tsarnaev had come up empty. They had said they did not know where the fugitive was, but would still lift a “shelter in place” order anyway.

“You can get back out, as long as you are vigilant,” said Massachusetts Gov. Deval Patrick (D).

As night fell again here, the Tsarnaevs had become a new lesson in the awful magnifying power of terrorism. Two unremarkable brothers, armed with low-tech bombs and no apparent escape plan, had allegedly killed four people, injured more than 170, and then held one of America’s great cities in terror for a full day. And counting.

While law enforcement officers continued to look for Tsarnaev, investigators in Washington and elsewhere began seeking to understand what had turned them to violence.

The Tsarnaev brothers are of Chechen heritage. Both were born in the Caucusus region, a cauldron fought over by Chechen separatists, Russian security forces, Islamic extremists and organized crime. They had emigrated legally, and lived for years in the Boston area, where their father, Anzor, was an auto mechanic.

In the past, both men had embraced American passions, according to friends and neighbors. Tamerlan, 26, was an accomplished boxer, with a wife and child. Dzhokhar was a wrestler at Cambridge’s public high school, who went on to attend the University of Massachusetts-Dartmouth.

On Thursday evening, authorities released photos of two men spotted carrying backpacks near the Marathon’s finish line. These were apparently the Tsarnaevs, though authorities did not say that at the time--and, perhaps, did not know it at all.

Their targets, it turned out, had not fled the city or the country. A few hours later, they began a violent spree just across the Charles River in Cambridge.

At about 10:30 p.m., authorities said, the two shot and killed MIT Police officer Sean Collier, 26, as Collier sat in his cruiser. It was unclear what triggered that shooting: authorities said surveillance video appears to show the two approaching Collier and killing him without warning.

After that, the men allegedly carjacked a Mercedes sport utility vehicle, and took the driver with them. At least one brother told the carjacked driver that they were the marathon bombers. They forced the driver to stop at several bank machines, and took $800 that he withdrew. After a few minutes, the man was left behind at a gas station, unharmed. It was unclear if he escaped, or was let go.

From there, the brothers drove about three miles, throwing explosives out the window at police. In Watertown, they engaged in a shootout with police, in which at least 200 rounds were fired.

Authorities said that Tamarlan Tsarnaev left the car at one point, and attempted to throw an explosive at officers. The device, however, went off in his hand. Police then tackled the wounded suspect in the street. Then, authorities said, Dzhokhar Tsarnaev--still behind the wheel of a car, swerved at the officers in an effort to hit them.

The officers dodged. Tamerlan did not. He was dragged under the car, and later died at a Boston hospital. In the same confrontation, a Boston Transit police officer was also shot and wounded. {{Officials later identified the officer as Richard H. Donahue, 33, who was treated at Mt. Auburn Hospital and released.

Dzhokhar Tsarnaev escaped. Police later conceded that there were not enough officers to establish a perimeter. The teenager is believed to have fled on foot: police found a small blood trail, indicating he was injured.

Then he vanished.

Starting after midnight Friday, an army of police officers and federal investigators began fanning out to look for him. SWAT teams from as far away as Cape Cod and New Hampshire rolled through the streets in armored cars, as police carefully searched house to house in Watertown.

While they looked, authorities issued a massive order to stay at home. It covered the entire city of Boston, plus the suburban towns of Belmont, Brookline, Cambridge, Newton, Waltham and Watertown.

“This situation is grave. We are here to protect public safety,” Boston Police Commissioner Ed Davis said in the morning, as the extraordinary lockdown was put in place. “We believe this to be a terrorist. We believe this to be a man here to kill people.”

As the afternoon wore on, authorities closed down the suspects’ street, and searched their home. President Obama was briefed. The Tsarnaev brothers’ relatives appeared one after another on cable TV, with one uncle denouncing the two as “losers.”

In Boston, a few people began to defy orders, and come out in the streets. As night fell, the governor said that everyone could do that--but with the frightening knowledge that an alleged killer and bomb-maker was out there, too.

“We can return to living our lives,” Patrick said.

While authorities looked for Tsarnaev, another phase of the investigation was already underway. In Washington and elsewhere, investigators interviewed family members, and searched for travel records, financial documents and for the brothers’ traces on Internet.

For now, the brothers’ alleged motive remains unknown. The Tsarnaevs, like many ethnic Chechens, are Muslim. In the last several months, Tamerlan Tsarnaev had posted videos to YouTube indicating his interest in radical Muslim ideologies.

Another question: did the men have help, either from within the U.S. or abroad? While militant Chechen groups have been blamed for terror attacks in the past, their target has usually been Russia, a bitter foe from Caucusus wars.

On Friday, U.S. authorities said they had no proof that anybody beyond the two Tsarnaev brothers was involved in the marathon attacks. But they were not done looking.