The two brothers captured on camera leaving the Boston marathon
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.
The suspect was captured on Friday evening after a huge manhunt during which Dzhokhar Tsarnaev's 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.
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.
'Throw the book at him'
It is unclear when the charges will be filed against the suspect.
In addition to the federal charges, prosecutors for the state of Massachusetts, which does not have the death penalty, may file their own.
Mayor Menino said he hoped the federal prosecutor for Massachusetts, Carmen Ortiz, "takes him [Dzhokhar Tsarnaev] on the federal side and throws the book at him".
Interrogators are not reading Mr Tsarnaev his Miranda rights, which guarantee the right to remain silent and the right to a lawyer.
This exception is allowed on a limited basis when the public may be in immediate danger.
Boston police commissioner Ed Davis said on Sunday he believed the brothers had probably been planning further attacks.
The federal public defender's office in Massachusetts has agreed to represent Mr Tsarnaev once he is charged.
He is being treated in Boston's Beth Israel Deaconess Medical Center for injuries he sustained before his capture, when he was found hiding in a boat in the backyard of a house in Watertown, a suburb of Boston.
Watertown's police chief, Ed Deveau, has said he believes Dzhokhar Tsarnaev mortally injured his brother just after their firefight with police.
It was initially reported that Tamerlan Tsarnaev, 26, had died of bullet and blast injuries.
Mr Deveau told the Boston Globe newspaper, however, that Dzhokhar had driven over him in a stolen SUV, dragging him on the pavement and apparently inflicting the injuries that killed him.
After Tamerlan shot at police and apparently ran out of bullets, the police chief said, officers tackled him.
They were trying to apply handcuffs when the SUV came roaring at them, with Dzhokhar at the wheel. The officers scattered and the SUV ran over Tamerlan Tsarnaev, Mr Deveau said.
Abandoning the car, Dzhokhar Tsarnaev fled the scene on foot, he said.
The same newspaper reports that Tamerlan Tsarnaev disrupted a mosque in Cambridge, Massachusetts, in January when he objected to the speaker comparing the Prophet Muhammad to civil rights champion Martin Luther King.
He reportedly told the speaker "You are a kafir [unbeliever]", and said he was contaminating people's minds and was a hypocrite.
Separately, US lawmakers on Sunday questioned why the FBI had failed to spot the danger from Tamerlan Tsarnaev after Russia had asked the US agency to question him two years ago.