Dzhokhar Tsarnaev, 19, was formally charged with using a weapon of mass destruction and malicious destruction of property resulting in death at a bedside hearing on Monday in his hospital room, where he was recovering from gunshot wounds suffered in shootouts with police.
Prosecutors say he and his elder brother Tamerlan Tsarnaev, 26, carried backpacks containing pressure cooker bombs that ripped through the crowd near the finish line of the world-renowned race on April 15, killing three people and wounding more than 200.
Tamerlan Tsarnaev was shot in an exchange of gunfire with police and run over by his younger brother early on Friday, police said. He later died at a hospital. Dzhokhar Tsarnaev fled on foot but was captured later that day following a massive manhunt.
Russian authorities flagged the elder Tsarnaev in 2011 as a possible Islamist radical, and some lawmakers have accused the FBI of failing to act thoroughly enough after Russia's security services raised their concerns. The FBI questioned him in 2011. Top investigators were slated to brief the full House of Representatives on Tuesday about the failure to spot the danger.
Dzhokhar Tsarnaev told investigators in his hospital room that he and his brother acted alone, without any help, according to reports by CNN and the New York Times. He said his older brother was the driving force behind the bombings, according to CNN.
The Times reported that he admitted to the bombings in questioning by investigators on Sunday. These reports could not be independently confirmed.
The younger Tsarnaev, an ethnic Chechen who is a naturalised US citizen, will be tried in a civilian rather than a military court. Some Republican lawmakers had called on the Obama administration to designate him as an enemy combatant, which would have restricted his rights.
Evidence against him is now likely to be presented to a grand jury by prosecutors who will seek a formal indictment. Both charges against Tsarnaev carry the possibility of the death penalty. Given the apparent evidence against him, plea negotiations are likely, legal experts said.
Emigrated from Dagestan
He can be seen in video taken by security cameras placing a backpack near the finish line of the marathon, according to the criminal complaint that alleges he acted in concert with his older brother.
The complaint drew from video and still images captured by security cameras, the media and the public at the race before and after the bombing.
The complaint, which did not mention a motive for the bombings, said that 30 seconds before the first explosion, Dzhokhar Tsarnaev started fidgeting with his cellphone. After the blast, virtually everyone around him turned toward the blast "in apparent bewilderment and alarm," while he appeared calm, it said.
He then left his backpack on the ground and walked away, the complaint said. About 10 seconds later the second explosion ripped through the crowd.
The Tsarnaev brothers emigrated to the United States a decade ago from Dagestan, a predominantly Muslim region in Russia's Caucasus.
The elder brother, a legal US resident, visited relatives in the volatile region of Chechnya for two days during his six-month trip out of the United States last year, his mother, Zubeidat Tsarnaeva and aunt, Patimat Suleimanova, told Reuters in Dagestan on Monday.
US authorities were investigating whether he became involved with Chechen separatists or Islamist extremists there. In his hospital bed, the younger Tsarnaev, a local college student, was alert and nodded on Monday when questioned by federal magistrate judge Marianne Bowler, according to a transcript of the bedside legal proceeding.
The bombings killed Krystle Campbell, a 29-year-old restaurant manager; Chinese graduate student Lingzi Lu, 23 and 8-year-old Martin Richard. A campus police officer at the Massachusetts Institute of Technology was shot and killed by the two brothers on Friday, police said.
As of Monday, Boston-area hospitals were still treating at least 48 people, with at least two listed in critical condition. Ten people lost limbs from the bombs packed with nails and ball bearings.
One of them, dance instructor Adrianne Haslet-Davis, interviewed from her hospital bed by CNN, lost her foot in the blast but vowed to dance again. She said she never lost consciousness and following the blast crawled on her elbows to seek help. "I remember everything," she told CNN.
Prosecutors will be deciding whether to seek the death penalty against Tsarnaev, and that decision is expected within weeks.
His capture capped a tense 26 hours after the FBI released the first pictures of the two bombing suspects, who were still unidentified, on Thursday.