LOS ANGELES (REUTERS) - A man identifying himself as the brother-in-law of one of the suspects in a deadly California shooting rampage said he has no idea what might have motivated the attack on a social services agency that left 14 people dead.

Farhan Khan, who said his sister was married to one of the people suspected of carrying out the massacre in San Bernardino, California, about 100 km east of Los Angeles, offered his condolences to the victims.

"Why would he do that? Why would he do something like this? I have absolutely no idea, I am in shock myself," Mr Khan said at a news conference in Anaheim, California, south of Los Angeles.

Two suspects - a man and a woman armed with assault rifles and handguns - were killed in a shootout with police hours after the attack, police said.

A third individual was detained while running from the scene of the shootout, but police said it was not immediately known whether that person was involved.

Mr Khan did not make clear which individual he was related to in the case.