US federal prosecutors are preparing charges against the surviving Boston Marathon bomb suspect Dzhokhar Tsarnaev as more details emerge of his capture.
If he is charged with using a weapon of mass destruction to kill people, he could face the death penalty.
Mr Tsarnaev is in hospital, unable to speak because of a wound to the throat.
US media quoted anonymous sources as saying he had been responding to questions in writing, but this has not been officially confirmed.
The FBI’s Boston field office and the Boston police department both said the information did not come from them.
Boston’s Mayor Tom Menino had earlier told ABC News that “we don’t know if we’ll ever be able to question the individual”.
But the ABC, NBC and CBS networks all reported late on Sunday that the suspect was responding in writing to interrogation. This included questions about possible cell members and other explosives.
“We have a million questions and those questions need to be answered,” said Massachusetts Governor Deval Patrick, quoted by Reuters news agency.
The suspect was captured on Friday evening after a huge manhunt during which his elder brother and suspected fellow bomber, Tamerlan Tsarnaev, died.
Police believe the 19-year-old Dzhokhar may have killed his brother himself, running him over in a car as he fled capture on Thursday night.
Monday’s twin bomb attack on the Boston Marathon finish line killed a boy of eight and two women, and injured more than 180, of whom 13 lost limbs.
One policeman was killed and another injured during the manhunt.
Governor Patrick has asked Bostonians to observe a moment of silence for the victims at 14:50 local time (18:50 GMT).
A funeral service will also be held on Monday for one of the victims, 29-year-old restaurant worker Krystle Campbell.
‘Throw the book at him’
No motive for the attack has been established. The brothers, who originate from Chechnya in southern Russia, had been living in the US for about a decade.