At least eight people have been killed when a transit employee opened fire at his co-workers at a light rail yard in San Jose, California, the county sheriff’s office says.

The gunman, who like the victims was an employee of the Santa Clara Valley Transportation Authority, was also dead, Santa Clara County Sheriff’s Deputy Russell Davis said on Wednesday (local time).

He did not say how the gunman died or whether police officers fired their weapons at the scene.

The name and age of the suspect were also not disclosed.

The first emergency calls reporting the shooting at the VTA light rail yard near the city’s main airport came through shortly after 6.30am local time.

A bomb squad was searching the yard after at least one explosive device was found, Mr Davis said.

“A horrible tragedy has happened today and our thoughts and love go out to the VTA family,” Glenn Hendricks, chairman of the VTA board, said at the news conference.

A law enforcement K-9 unit searches for evidence at the Valley Transportation Authority (VTA) light-rail yard. Photo: Getty

He said the shooting happened in a part of the yard where workers do maintenance on vehicles and not in the operations and control centre.

San Jose, a city with about 1 million residents, lies at the heart of Silicon Valley, a global centre of technological innovation and home of some of the biggest tech companies in the US.

“These are, and were, essential workers,” San Jose Mayor Sam Liccardo said.

“These VTA employees helped us get through this horrific pandemic. They were showing up everyday to operate light rail and buses to ensure people could still go about their lives in the middle of the challenge of the pandemic. And they were taking risks with their own lives in doing so.”

He said he was aware of news reports of a fire at the house of the man the authorities believed to be the shooter.

“That is certainly the information that I have, is that there was a fire at the shooter’s home, there was nobody found inside the home, thank God,” Mr Liccardo told CNN affiliate KGO.

“This is every mayor’s worst nightmare.”

The Federal Bureau of Investigation said it had agents assisting at the crime scene.

US President Joe Biden’s staff were monitoring the situation.

“Our hearts go out to the victims and their families,” White House spokeswoman Karine Jean-Pierre said.

-AAP