The FBI is to face questions in the US Congress over whether they mishandled information about Boston bombing suspect Tamerlan Tsarnaev.
The security officials will brief the Senate Intelligence Committee in a closed hearing, after some lawmakers accused the FBI of failing to act on Russian concerns.
Boston bomber Tamerlan Tsarnaev was questioned in 2011 amid claims he had adopted radical Islam.
Tamerlan Tsarnaev was killed in a manhunt after the attack but his wounded brother Dzhokhar has been charged over the bombings.
Federal prosecutors charged Dzhokhar Tsarnaev in hospital 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 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 Tsarnaev brothers 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.
The FBI is to face questions in the US Congress over whether they mishandled information about Boston bombing suspect Tamerlan Tsarnaev
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 Boston 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.
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 Tamerlan Tsarnaev and his family also revealed nothing suspicious.
However, 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 Lindsey Graham added that the US authorities did not know Tamerlan Tsarnaev had gone to Russia in 2012 because his name was misspelled in travel documents.
The suspect 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, 26, was killed during the police manhunt last Friday. Dzhokhar Tsarnaev, 19, 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.
Dzhokhar 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.
US Congress questions for the FBI:
Why was no further action taken after the 2011 investigation of Tamerlan Tsarnaev?
Why Tamerlan Tsarnaev was not identified as a threat based on links to radical websites?
Why were the authorities unaware of Tamerlan Tsarnaev’s visit to Russia in 2012?
Senior US Democrats Dan Malloy and Dianne Feinstein have called for stricter gun control following Sandy Hook Elementary School shootings in Newtown, Connecticut.
Twenty children and six teachers died in Friday’s assault on Sandy Hook school by a lone gunman who then turned his weapon on himself.
Connecticut Governor Dan Malloy said he wanted stronger national limits.
And Senator Dianne Feinstein said she would introduce a bill to ban assault weapons as soon as Congress convened.
A nationwide ban on certain semi-automatic rifles in the US expired in 2004.
President Barack Obama – who shortly after the school attack urged “meaningful action” against gun crime in the US – is to visit Newtown on Sunday.
Barack Obama will meet families and emergency service workers, and speak at an interfaith vigil at the town’s high school.
Ahead of his visit, reports from Newtown say a service for the victims at St Rose of Lima Roman Catholic Church has been abandoned and the church evacuated because of an unspecified threat.
The gunman behind Friday’s shootings has been named in media reports as Adam Lanza, who is said to have killed his mother before driving to the school, opening fire and then killing himself.
The state’s chief medical examiner said the gunman used a semi-automatic rifle as his main weapon, and all the victims appeared to have been shot several times, some of them at close range.
Speaking on Sunday, Governor Dan Malloy said Connecticut had an existing ban on assault weapons, but the lack of a similar law at federal level made it difficult to keep them out of the state.
“These are assault weapons. You don’t hunt deer with these things,” he told CNN.
“One can only hope that we’ll find a way to limit these weapons that really only have one purpose.”
Governor Dan Malloy had to break the news to most of the victim’s families on Friday.
“You can never be prepared for that – to tell 18 to 20 families that their loved one would not be returning to them that day or in the future,” he said.
Senator Dianne Feinstein, who represents California in the upper house of Congress and is a long-term supporter of stricter gun control, told US TV network NBC: “I’m going to introduce in the Senate, and the same bill will be introduced in the House (of Representatives), a bill to ban assault weapons.”
Asked if President Barack Obama would support her measure, she said: “I believe he will.”
Senior US Democrats Dan Malloy and Dianne Feinstein have called for stricter gun control following Sandy Hook Elementary School shootings in Newtown
New York Mayor Michael Bloomberg, another strong gun control advocate, has urged President Obama to act.
“We have heard all the rhetoric before,” he said.
“What we have not seen is leadership – not from the White House and not from Congress. That must end today.”
All 20 children who died in the shootings – eight boys and 12 girls – were aged between six and seven, according to an official list of the dead were killed. The youngest, Noah Pozner, had celebrated his birthday only last month.
The head teacher at Sandy Hook school in Newtown, Dawn Hochsprung, was among the dead, along with adults Rachel DaVino, Anne Marie Murphy, Lauren Russo, Mary Sherlach and Victoria Soto.
A woman who worked at the school was the only person to be shot and survive.
Scores of people have left tributes at a memorial outside the school, while a minute’s silence is being held before National Football League games across the US on Sunday.
Connecticut State Police say the process of releasing the victims’ bodies to their families is under way, and have condemned what they term “misinformation” being published on social media about the tragedy – including people wrongly claiming to be the gunman.
The authorities now say the gunman forced his way into the school, contradicting first reports that he was been let in voluntarily.
Investigators say they have gathered “good evidence” in the search for a motive, but have not given any details.
The gunman is said to have shot dead his mother at their home before driving to the school in her car and opening fire on the victims. Reports say the guns found at the scene were registered to her.
Education officials say they have found no link between the gunman’s mother and the school, contrary to earlier reports that said she was a teacher there, the Associated Press news agency reports.
Investigators said they believe Adam Lanza attended Sandy Hook many years ago.
The killings took place in two rooms and a hallway within a single section of the school, police have said. The shooting lasted just a few minutes.
As they heard the shots, teachers in other parts of the building tried to protect children by locking doors and ushering them into closets.
Police say that children are unlikely to return to the classrooms where the shootings took place. Surviving pupils will be taught at other schools in the area while a final decision about the Sandy Hook’s future is made, though one official said it was unlikely to re-open.
The suspected gunman’s father, Peter Lanza, said his family was “struggling to make sense of what has transpired”.
“Our family is grieving along with all those who have been affected by this enormous tragedy,” he said in a statement.
Pope Benedict XVI paid tribute to the victims and their families in his weekly address at the Vatican, saying he was “deeply saddened by Friday’s senseless violence”.
The attack at Newtown is the second deadliest shooting attack at a US school or university, after the Virginia Tech killings of 2007, which left 32 people dead and many injured.