Boston Marathon bombing: FBI quizzed over Tamerlan Tsarnaev
US security officials have been questioned by Congress over whether they mishandled information about the Boston bomb suspect Tamerlan Tsarnaev.
The Senate Intelligence Committee is investigating whether the FBI failed to act on Russian concerns that Tsarnaev was becoming radicalised.
Tsarnaev was questioned in 2011 amid claims he had adopted radical Islam.
At a closed hearing, senators said steps could be taken to improve information sharing between agencies.
Tsarnaev was killed in a manhunt after the attacks on 15 April. His brother Dzhokhar, was wounded but survived, and 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.
The 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.
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.
In 2012 he travelled to Russia, and spent six months in Dagestan, another mainly Muslim Russian republic bordering Chechnya. During the visit, he also reportedly spent two days in Chechnya itself.
Republican Senator Lindsey Graham had 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.
Speaking after the Intelligence Committee hearing in Washington, Republican Senator Susan Collins said there appeared to be "serious problems with sharing information, including critical investigative information ... not only among agencies but also within the same agency in one case", the Associated Press reports.
The vice-chairman of the committee, Republican Senator Saxby Chambliss, said he could not see "anybody yet that dropped the ball", but that he was still seeking information on whether information was properly shared.
"If it wasn't, we've got to fix this," AP quoted him as saying.
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.
It said 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.
In a separate hearing on Tuesday, Homeland Security Secretary Janet Napolitano said the FBI had been aware of Tamerlan Tsarnaev's trip to Russia, contradicting Senator Graham's allegation that the trip had been overlooked because his name had been misspelled in travel documents.
The twin bombs which exploded near the finishing line of the marathon killed three people and injured more than 200, many of them seriously.
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.
On Monday, a 10-page criminal complaint was filed against Dzhokhar Tsarnaev during a court hearing around his hospital bed.
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.
According to a transcript of the hearing, he managed to speak once despite a gunshot wound to his throat sustained during his capture.
Mr 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.
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 were 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."