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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. An MIT officer was allegedly killed by Tsarnaev and his brother Tamerlan and a Boston transit cop was badly wounded in a subsequent shootout.

The FBI is still collecting evidence from the Monday's blast site, mainly from the sides of buildings and the bleachers.

Boston Police Commissioner Ed Davis in a news conference Sunday that the circuit board to at least one of the bombs had wires crudely soldered to it and is confident ATF 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 develop a plan to reopen Boyleston Street now the most famous street in the world," Menino said at the Sunday news conference.