An elite interrogation team stood ready to question the surviving suspect in the Boston Marathon bombings Saturday as other investigators aggressively hunted clues to the terrorists' motives and possible connections.
While Dzhokhar Tsarnaev, 19, recovers from his wounds under guard in a Boston hospital, federal prosecutors are preparing charges against him, which could include murder, committing an act of terrorism and the use of weapons of mass destruction -- in this case, bombs, a federal law enforcement source said. Prosecutors could decide to seek the death penalty.
He was still unable to talk Saturday, officials said.
"We have a million questions, and those questions need to be answered," said Gov. Deval Patrick, speaking outside Fenway Park.
Investigators are scrutinizing a half-dozen "active scenes," including the marathon bomb site and the boat stored outside a suburban house where Tsarnaev was found hiding Friday night and captured after a shootout, authorities said. "This thing has not lost any steam," said Special Agent Daniel Curtin of the FBI's Boston office. "In fact it's picked up steam."
Dzhokhar Tsarnaev and his brother, Tamerlan Tsarnaev, 26, are believed to have set off two pressure-cooker bombs Monday, killing three and wounding 176, authorities said.
Dzhokhar, bloody when he was found and suffering from a neck wound, was in serious condition at Beth Israel Deaconess Medical Center, which cared for 24 patients after the bombings. Eleven were still there, one in serious condition.
Looking for links to groups
Investigators looked for links between the brothers, with roots in the troubled Chechnya region of Russia, and extremist groups abroad. They collected clues at the multiple crime scenes, including the Watertown street where the manhunt ended.
In New Bedford, an hour's drive south of Boston, two men were picked up Saturday for questioning at an apartment complex in connection with the investigation surrounding the surviving bombing suspect. The two were among three people picked up for questioning -- and released -- at the same apartment complex Friday.
The surviving bomber once lived with the men, a federal law enforcement source said.
More details of the brothers' actions after the marathon bombings emerged.
"They could have attempted to go into hiding," the federal source said. "A choice was made to do the opposite" -- a new spasm of violence that included a carjacking, a hostage- taking and the "assassination-like" killing Thursday night of rookie MIT cop Sean Collier, 26.
He became the fourth to die from the terror spree. Martin Richard, 8; Krystle Campbell, 29, a restaurant manager; and Lu Lingzi, 23, a Boston University graduate student from China, were killed in the marathon blasts.
Slain suspect was probed
Once the FBI learned the brothers' identities, it scoured its records, finding that in early 2011, the Russian government asked the agency to investigate Tamerlan Tsarnaev, two law enforcement officials told The Associated Press.
The request was based on information that he was a follower of radical Islam, the FBI said.
The FBI said it interviewed Tamerlan Tsarnaev and his family members and did not find any terrorist ties, domestic or foreign.
But the federal law enforcement source said the FBI is also looking at trips the older brother made to Chechnya and Dagestan as it investigates his possible links to terrorist and extremist groups in Russia and former Soviet satellite countries.
Ruslan Tsarni of Montgomery Village, Md., the brothers' uncle, said Tamerlan told him in a 2009 phone conversation that he had chosen "God's business" over work or school.
Then a family friend told him Tsarnaev had been influenced by a recent convert to Islam. Tsarni said his relationship with his nephew ended after that call.
Tsarnaev's social media presence suggested interest in or affinity with jihadists and radical clerics. He created a playlist and "liked" several videos with a jihadist theme, according to the SITE Intelligence Group, which studies international terrorists.
Tsarnaev's mosque in Cambridge, the Islamic Society of Boston, said in a statement that "Our community and people in the institutions above, are in shock to have learned the crimes of these individuals."
Mosque-goers Saturday recalled an incident when Tamerlan Tsarnaev objected to a sermon comparing Martin Luther King Jr. -- a non-Muslim -- to the Prophet Muhammad.
"At that point, he stood up and took offense at that," said Yusufi Vali, executive director of an affiliated mosque in Roxbury. Still, Vali said, "There was nothing in his behavior that suggested any kind of violence."
Vali said the younger brother came there only once.
Cops saw them in 2 cars
The federal source with information about the investigation said that as the brothers were fleeing from the cop killing early Friday, a police officer saw them driving two vehicles -- one behind the other -- in Watertown.
The men saw the officer's police car, got out of their vehicles and opened fire. At one point, Tamerlan Tsarnaev detonated a pressure cooker bomb, sources said. A transit cop was seriously wounded in the mayhem.
Mike Doucette, 27, of Laurel Street, watched the shootout from his windows, with the police and suspects exchanging some 200 rounds.
Though he couldn't identify the brothers at the time, he saw a man later identified by police as Dzhokhar throw the bomb -- he said it looked like a "huge fireball" -- that exploded in midair. A man later identified as Tamerlan walked westbound on Laurel Street toward the police, who yelled at him to stop.
Tamerlan ran out of ammunition after about five minutes, at which point police rushed him and got him on the ground. During the struggle, Dzhokhar drove off in the Mercedes SUV, running over his brother, the witness said and a source confirmed. Doucette could see his body "tumbling underneath the vehicle," coming to rest in front of his driveway.
Doctors who treated Tamerlan Tsarnaev said he suffered gunshot wounds as well as those consistent with trauma from an explosion, but it's not clear which of the devices caused his injuries or when.
As investigators pressed on, so did Bostonians.
At the first baseball game at Fenway since the bombings, the Red Sox honored the victims, survivors and heroes with a pregame ceremony and an emotional video of scenes from Monday's race.