(Yahoo!) -- Boston Marathon bombing suspect Dzhokhar Tsarnaev is awake and "responding sporadically in writing to questions ... about other cell members and other unexploded bombs, " law enforcement sources told ABC News on Sunday evening.
Authorities had been waiting to question the 19-year-old, who is accused of helping to carry out the attack, which killed three people and wounded 180 others, many of them critically, near the finish line of Monday's race.
Earlier Sunday, the FBI said Tsarnaev remains in serious condition at the heavily-guarded Beth Israel Deaconess Medical Center, and a federal official told Yahoo News that Tsarnaev was sedated and unable to speak.
Tsarnaev was brought by ambulance to the facility after he was captured in Watertown, Mass., on Friday, following an intense manhunt that included at least two shootouts with police and ended with the bloodied suspect taken into custody from a tarp-covered boat he had been hiding in. He apparently suffered gunshot wounds to the neck and leg.
Tsarnaev's 26-year-old brother, Tamerlan, the other suspect wanted by the FBI, was killed during a late-night firefight with police in Watertown. Tsarnaev managed to escape on foot, prompting a citywide lockdown as police conducted a house-by-house search for the alleged killer.
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The Tsarnaev brothers, who were born in the former Russian territory known as Kyrgyzstan and are of Chechen descent, lived in Cambridge, Mass., for several years. Dzhokhar became a naturalized American citizen last year.
Under U.S. law, authorities have 72 hours after Tsarnaev's arrest to file a criminal complaint against him. When they do, Tsarnaev is expected to face terror charges that could bring the death penalty.
"I hope that the U.S. attorney, Carmen Ortiz, takes him on the federal side and throws the book at him," Boston Mayor Thomas Menino said on ABC's "This Week With George Stephanopoulos" on Sunday.
Richard Roper, a former U.S. District Attorney in the Northern District of Texas, told Yahoo NewsDzhokhar could be charged in his hospital room.
“Then when he’s better, he can make an appearance in court,” Roper said.
Hours after the FBI made a public plea to help identify the suspects from video and photos at the marathon, the brothers allegedly shot and killed an MIT police officer and wounded a transit cop. Authorities said the pair launched explosives during a dramatic high-speed chase in residential Watertown early Friday.
No motive has been revealed for Monday’s attack.
Immediately after Dzhokhar’s capture, federal prosecutors stirred controversy in legal circles by refusing to grant Dzhokhar his Miranda rights against self-incrimination, citing public safety concerns.
"He is not going to be read the Miranda warnings," ABC News Senior Justice Correspondent Pierre Thomas said Sunday. "They are going to use the Public Safety Exception, and dive in without advising him of his right to remain silent. They are taking this extraordinary step because there could be an imminent threat still out there. ... There's deep, deep concern about the amount of ammunition, guns and working bombs these men had."
Boston Police commissioner Ed Davis told "Fox News Sunday" that the amount of explosives police found suggested that the brothers may have been planning another attack.
"We are hoping, for a host of reasons, that the suspect survives," Massachusetts Gov. Deval Patrick said Saturday. "Because we have a million questions, and those questions need to be answered."