Image caption Tamerlan Tsarnaev travelled to Russia in 2012 for a lengthy trip to the Russian republic of Dagestan

One of the Boston bomb suspects was added to a terrorism database 18 months ago at the request of the CIA, officials have told US media.

The FBI has already said it investigated Tamerlan Tsarnaev, 26, but had found no evidence of a threat.

Tsarnaev was killed during a police chase last week. His brother Dzhokhar, 19, is in custody over the bombs.

Three people were killed and more than 260 wounded when two devices exploded at the Boston Marathon on 15 April.

A US politician earlier confirmed the bombs were set off by remote control.

But the devices were not sophisticated and apparently had to be triggered from a few streets away.

FBI 'not at fault'

Officials said Tamerlan Tsarnaev had been added to the Terrorist Identities Datamart Environment (Tide) on the request of the CIA.

The database contains as many as 745,000 entries, and individuals on that list are not necessarily on the so-called terrorist watch list.

The Tsarnaev brothers Sons of Chechen refugees from the troubled Caucasus region of southern Russia

Family is thought to have moved to the US in 2002 from Russian republic of Dagestan

They lived in the Massachusetts town of Cambridge, home to Harvard University

Dzhokhar, 19, was awarded a scholarship to pursue further education; he wanted to become a brain surgeon, according to his father

Tamerlan, 26, was an amateur boxer who had reportedly taken time off college to train for a competition; he described himself as a "very religious" non-drinker and non-smoker Q&A: Suspects' Chechen links

The Russian authorities had alerted US counterparts to the activities of Tamerlan Tsarnaev, whose family has its origins in the war-torn Russian republic of Chechnya.

About six months before the CIA requested his name be added to Tide, the FBI asked the Russians for more information about Tamerlan Tsarnaev but received none, and closed its investigation.

The authorities earlier said the US intelligence community had no information about threats to the marathon ahead of the 15 April attacks.

After a classified briefing in the House intelligence committee on Wednesday, Democratic Congressman Dutch Ruppersberger said he believed the FBI was not at fault.

"I feel, based on the testimony today, that the FBI did exactly what they would do and they followed through the protocols that were necessary once they got that information," Mr Ruppersberger told reporters.

He also said he had been told the bombs were detonated with a "garage door opener-type of device".

Dzhokhar Tsarnaev was injured during the police manhunt and remains in hospital in a fair condition.

Suspects' parents arriving

Officers captured him as he hid in a boat covered by a tarpaulin in a garden in Watertown, Massachusetts.

Media playback is unsupported on your device Media caption Congressman Dutch Ruppersberger: ''The FBI... followed through the protocols"

Officials initially had said he exchanged gunfire with police for more than an hour before he was captured on Friday.

But the Associated Press quoted two unnamed officials as saying Dzhokhar Tsarnaev had been unarmed when he was captured.

The younger brother has been charged 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.

In bedside questioning, Dzhokhar Tsarnaev has said he and his brother were angry about the US wars in Iraq and Afghanistan.

But the brothers are not believed to have had direct contact with a militant organisation, politicians said after closed-door briefings.

Media playback is unsupported on your device Media caption Russia's rebellious Caucasus republics have been spawned war and insurgency for the past 20 years

It is suspected the brothers became radicalised online.

The suspects' parents, Anzor Tsarnaev and Zubeidat Tsarnaeva, are due to arrive in the US on Thursday, Russian media reported.

The Tsarnaev family has origins in the predominantly Muslim republic of Chechnya in southern Russia.

The brothers had been living in the US for about a decade at the time of the attack.

In 2012, Tamerlan Tsarnaev spent six months with relatives in Dagestan, another Russian republic, which has an Islamist militant insurgency.