Boston Marathon bombing suspect Tsarnayev is responding to questions in writing

Boston Marathon bombing suspect Dzhokhar Tsarnayev is awake and responding sporadically in writing to questions about other cell members and other unexploded bombs, law enforcement sources told ABC News.

Previously officials said Tsarnayev was in no condition to be interrogated.

Tsarnayev, 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 Tsarnayev 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.