Accoridng to the BBC report, the attack took place at an event on Wednesday for employees of the San Bernadino public health department.  None of the victims has been identified.

The Inland Regional Center, where the shooting took place, provides services for people with developmental disabilities, but the shooting appeared to be unrelated to its clients, BBC said.

The incident marks the deadliest mass shooting in the US since 26 people were killed at a school in Newtown, Connecticut in 2012.

Mr Burguan, who said another 17 people were wounded, said police were "reasonably confident" that there were just two shooters.  A third person seen running from the scene was detained, but police said it was unclear whether that person was involved.

Both suspects had been  killed in their vehicle after a shoot-out with police involving 20 officers. A tip-off had earlier led police to a house in nearby Redlands.

Earlier, the FBI said it was not ruling out the possibility of terrorism but the situation was still being investigated.

San Bernardino is a city of about 200,000 people, 60 miles (100 km) east of Los Angeles.