A California transit employee killed eight co-workers and wounded another before taking his own life on Wednesday, the latest in a spate of deadly US mass shootings, prompting the state's governor to ask: "What the hell is wrong with us?"

Authorities did not immediately offer many details or a possible motive for the shooting, which unfolded about 6:30 am Pacific Time (1330 GMT) at a light-rail yard for commuter trains of the Santa Clara Valley Transportation Authority (VTA), in the heart of Silicon Valley in the San Francisco Bay Area.

A bomb squad was searching the yard and adjacent buildings after at least one explosive device was found, Santa Clara County Sheriff's Deputy Russell Davis said at a news conference.