Tsarnaev, 19, was pulled, wounded and bloody, from a tarp-covered boat in a Watertown backyard Friday. He was taken into custody at the end of a tense day that began with his 26-year-old brother, Tamerlan, dying in a gunbattle with police.
Tsarnaev on Saturday lay hospitalized in serious condition under heavy guard at Beth Israel Deaconess Medical Center, the same place where his brother died. As investigators tried to establish the motive for the deadly attack and the scope of the plot, Tsarnaev was apparently in no shape to be interrogated.
No charges have been filed against Tsarnaev. There was no immediate word on when Tsarnaev might be charged and what those charges would be. The twin bombings killed three people and wounded more than 180.
The brothers' uncle said Tamerlan Tsarnaev had great influence over his younger brother.
"I may believe he's been full of evil, maybe he's been himself as an evilHe turned to be an evil. As I said, confused, entirely confused," said Ruslan Tsarni.
President Barack Obama said there are many unanswered questions about the marathon bombing, including whether the Tsarnaev brothers - ethnic Chechens from southern Russia who had been in the U.S. for about a decade and lived in the Boston area - had help from others.
The president urged people not to rush judgment about their motivations.
"Through days that would test even the sturdiest of souls, Boston's spirit remains undaunted," Mr. Obama said.
The FBI acknowledges it interviewed Tamerlan Tsarnaev in 2011 on a tip from Russia. They said he was a radical Islamist preparing to leave the U.S. After looking into his phone records and Web sites he visited, the FBI found no ties to terrorists and gave up the search.
Saturday marked the first Boston Red Sox game at Fenway Park since the bombing. The team's home jersey read "Boston" instead of "Red Sox." Singer Neil Diamond sang Fenway favorite, "Sweet Caroline."
The city's other professional sports teams showed their support as well. The Celtics wore a Boston Marathon patch on their jersey, while the Bruins wore hats honoring local police.
A candlelight vigil was held Saturday night for Sean Collier, the Massachusetts Institute of Technology officer allegedly gunned down by the two brothers in the midst of their wild crime spree.
The Associated Press contributed to this report.