FBI agents are assisting Kenyan officials in the investigation into a large explosion in downtown Nairobi, a Kenyan official says.

An explosion ripped through a building full of small shops on Monday, injuring at least 33 people, including a woman who blamed the blast on a "bearded man" who left behind a bag shortly before the detonation.

A Kenyan official who said he could not be quoted confirmed the FBI's involvement on Tuesday.

The intelligence firm IntelCenter said that militants from the al-Qaida-linked Somali group al-Shabab bragged about conducting interviews of survivors after the blast. IntelCenter said the development poses significant challenges to security forces and legitimate members of the media covering attacks.