NBC News’ Pete Williams reported that Tsarnaev, 19, is beginning to respond to a team of federal investigators mainly through writing. Williams reported that he is being questioned for brief periods and that he has difficulty communicating because of his throat injuries.

Tsarnaev was listed in serious but stable condition at a Boston hospital where he has remained since being detained by authorities on Friday, NBC said. He has been intubated and sedated, according to reports.

ABC News reported that investigators are asking about other possible terrorist cell members as well as about undetected explosives as they attempt to determine the motive for the deadly attack.