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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, according to BBC. 

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 within a few streets of the explosives.

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 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.