(BOSTON) -- Boston Marathon bombing suspect Dzhokhar Tsarnaev is awake and responding sporadically in writing to questions, authorities said.
Investigators are asking about other cell members and other unexploded bombs, law enforcement sources told ABC News.
Previously, officials said Tsarnaev was in no condition to be interrogated.
Tsarnaev, 19, is being treated at Beth Israel Deaconess Medical Center in Boston, where he is listed in serious but stable condition, with wounds to the neck and throat area, according to sources.
The bombing killed three, including a young boy, and wounded about 170 others. An MIT officer was allegedly killed by Tsarnaev and his brother Tamerlan, 26, and a Boston transit cop was badly wounded in a subsequent shootout.
The FBI is still collecting evidence from Monday's blast site, mainly from the sides of buildings and the bleachers.
Boston Police Commissioner Ed Davis said in a news conference on Sunday that the circuit board to at least one of the bombs had wires crudely soldered to it. He is confident that Bureau of Alcohol, Tobacco, Firearms and Explosives agents will put that bomb back together piece by piece and tell us what it was made of.
Engineers are also checking the structural integrity of buildings near the scene before it fully reopens. Boston Police plan to start returning personal items to people who left behind their things on Monday.
Boston Mayor Thomas Menino said it is time for the city to move forward.
"I'm working hard over the last several days to develop a plan to reopen Boyleston Street now the most famous street in the world," Menino said at the Sunday news conference.
Tsarnaev could face charges at the state and federal levels, but Massachusetts has no death penalty.
Menino hopes federal authorities "throw the book at him," which would include the possibility of execution.
"I hope that the U.S. attorney takes him on the federal side and throws the book at him," Menino said on ABC's This Week. "These two individuals held this city hostage for five whole days."
"They should not do that -- that's what these terrorist events want to do, hold the city hostage and stop the economy of the city," he added.
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