US security officials are to face questions in Congress over whether they mishandled information about Boston bombing suspect Tamerlan Tsarnaev.
They will brief the Senate Intelligence Committee in a closed hearing, after some US lawmakers accused the FBI of failing to act on Russian concerns.
Tsarnaev was questioned in 2011 amid claims he had adopted radical Islam.
He was killed in a manhunt after the attack but his wounded brother Dzhokhar has been charged over the bombings.
The Beth Israel Deaconess hospital in Massachusetts said at noon on Tuesday that the surviving brother’s condition had improved from “serious” to “fair”, according to the US Attorney’s Office in Boston.
Federal prosecutors have charged him with using a weapon of mass destruction and malicious destruction of property resulting in death. He could be sentenced to death if convicted on either count.
Anonymous officials have told US media that 19-year-old Dzhokhar Tsarnaev said he and his brother had planned the attack themselves without help from foreign militants.
The officials say his written answers from his hospital bed to investigators’ questions lead them to believe that the pair was motivated by jihadist ideology and that they devised the bombings using the internet.
However, the sources also said the interviews were preliminary and they must verify the defendant’s responses.
Lawyers for Katherine Russell, the widow of 26-year-old Tamerlan Tsarnaev, said that their client was doing everything she could to assist authorities.
She is “trying to come to terms with these events”, her lawyers said in a statement on Tuesday, without saying whether she had been questioned by investigators.
“The report of involvement by her husband and brother-in-law came as an absolute shock to them all.”
Both Tsarnaev brothers had origins in the troubled, predominantly Muslim republic of Chechnya in southern Russia. They had been living in the US for about a decade at the time of the attack.
The twin bombs which exploded near the finishing line of the marathon killed three people and injured more than 200.
Of those injured, 13 lost limbs. More than 50 people remain in hospital, three of them in a critical condition.
On Tuesday, Boston Mayor Thomas Menino announced that a compensation fund for victims of the attack had received $20m (£13.2m) in the week since it was launched, with donations streaming in from Boston and across the world.
Members of Congress want to know why no further action was taken after Tamerlan Tsarnaev was investigated in 2011 at the request of the Russian government.
Democratic Senator Dianne Feinstein, who chairs the intelligence committee, said that she and her colleagues would have to “sort it out” when they met FBI officials later on Tuesday.
The full Senate is expected to receive a briefing later in the week.
The FBI has defended itself, saying in a statement on Friday that it had run checks on the suspect but found no evidence of terrorist activity.
A request to Russia for further information to justify more rigorous checks went unanswered, and an interview by agents with Tsarnaev and his family also revealed nothing suspicious.
But Republican Senator Lindsey Graham questioned why the FBI was unable to identify him as a threat based on his alleged links to radical websites.
He called for better co-operation with Russia and the amendment of privacy laws to allow closer scrutiny of suspects’ internet activity.
Senator Graham added that the US authorities did not know Tsarnaev had gone to Russia in 2012 because his name was misspelled in travel documents.
He spent six months in Dagestan, another mainly Muslim Russian republic bordering Chechnya. During the visit, he also reportedly spent two days in Chechnya itself.
Tamerlan Tsarnaev was killed during the police manhunt last Friday. Dzhokhar Tsarnaev was captured later that day and remains in hospital with serious injuries.
A 10-page criminal complaint was filed against Dzhokhar Tsarnaev on Monday during a court hearing held around his hospital bed.
According to a transcript of the hearing, he managed to speak once despite a gunshot wound to his throat sustained during his capture.
Tsarnaev said the word “no” when asked if he could afford a lawyer. Otherwise he nodded in response to Judge Marianne Bowler’s questions from his bed at Boston’s Beth Israel Deaconess Medical Center.
The next hearing in his case has been scheduled for the end of May.
The complaint seeks to locate both suspects at the scene of the bombing and then pieces together the operation to intercept them three days later, as they allegedly drove a hijacked car near the city, hours after images of their faces were broadcast by the media.
No mention is made of their possible reasons for attacking the marathon.