SAN JOSE, CALIFORNIA (REUTERS) - A transit employee shot eight co-workers to death and was himself killed at a commuter rail yard in San Jose, California, on Wednesday (May 26), authorities said, 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 6.30am Pacific Time (9.30pm Singapore time) at a light-rail yard of the Santa Clara Valley Transportation Authority (VTA).

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

Davis did not say how the assailant died or whether police officers called to the scene had fired their weapons.

Sheriff Laurie Smith told reporters that gunfire was still going on when her first deputies arrived on the scene, and that the assailant took his own life, apparently when he became aware that police were closing in on him.

Officers never exchanged gunfire with the suspect, Davis said. Still, Smith credited swift action by sheriff's deputies, who rushed to the scene from their own headquarters next door to the rail yard, preventing what might have been a much greater loss of life.

Governor Gavin Newsom, appearing with Smith and others in San Jose, voiced exasperation at the frequency and regularity of gun violence in America.

"There is a sameness to this and that numbness, I think, is something that we are all feeling," Newsom said.

"It begs the damn question, What the hell is going on in the United States of America? What the hell is wrong with us and when are we going to come to grips with this?"

The gunman, the eight victims shot dead, and a survivor who was hospitalised in critical condition were all employees of the transit agency situated near the city's airport, officials said.

The victims were found in two buildings on the site. The County of Santa Clara medical examiner-coroner's office identified the victims late on Wednesday. They appeared to all be men and ranged in age between 29 and 63. Their names are: Paul Delacruz Megia, Taptejdeep Singh, Adrian Balleza, Jose Dejesus Hernandez III, Timothy Michael Romo, Michael Joseph Rudometkin, Abdolvahab Alaghmandan and Lars Kepler Lane.

San Jose's mayor, Sam Liccardo, said authorities also had responded to a fire at the home of the suspect, though no one was found inside.

The San Jose Mercury News and other media outlets identified him as Samuel Cassidy, 57, a maintenance worker at the yard.

Cassidy had worked for the transit authority since at least 2012, when he was listed as an "electro-mechanic," and was promoted to "substation maintainer" in 2015, according to records posted by the nonprofit website Transportation California.

Last year, he earned a salary of US$102,000, plus benefits and US$20,000 in overtime, the records showed.

Local television station KTVU-TV reported that Cassidy had been married for 10 years and divorced in 2009, citing an interview with his ex-wife, who reportedly told the station she had little contact with him over the past decade.

'Essential workers'

"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.

He said the shooting took place in a section of the rail yard where workers perform maintenance on vehicles, and was not in the facility's operations and control center.

San Jose, a city of about one million residents, lies at the heart of Silicon Valley, a global technology hub and home to some of America's biggest high-tech companies.

"These are, and were, essential workers," Liccardo said of the victims.

"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," the mayor said.

Multiple fire department, police and bomb squad vehicles were still parked outside the suspect's house, along a cul-de-sac in southeastern San Jose, hours after the shooting.

An explosives-detecting robot sat in the street near the home while two bomb squad technicians entered the ranch-style house. Arson investigators along with agents of the FBI and US Bureau of Alcohol, Tobacco, Firearms and Explosives were also present.

President Joe Biden was briefed on the shooting, and his staff continued to monitor the situation while keeping in close contact with local officials to offer any assistance needed, the White House said.

"What's clear, as the president has said, is that we are suffering from an epidemic of gun violence in this country, both from mass shootings and in the lives that are being taken in daily gun violence that doesn't make national headlines," White House spokeswoman Karine Jean-Pierre told reporters.

Mass gun violence, commonplace in a country with one of the highest rates of firearm ownership in the world, has escalated considerably following a year-long lull as the United States emerged from the worst of the Covid-19 pandemic this spring.

Wednesday's incident was the latest of at least nine deadly US mass shootings that have made national headlines in the past three months, starting with a string of attacks at Atlanta-area day spas in mid-March that claimed eight lives, and a rampage days later that killed 10 at a Colorado supermarket.

Last month, a former employee of an Indianapolis FedEx center shot eight workers to death before taking his own life.

Earlier this month a man fatally shot his girlfriend and five other people before taking his own life at a birthday party in Colorado.

The United States saw at least 200 mass shootings in the first 132 days of this year, according to a report by the Gun Violence Archive, a non-profit research group that defines such incidents as those in which four or more people, other than the assailant, are shot, regardless of the number killed.

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," Liccardo told CNN affiliate KGO in an interview.

"This is every mayor's worst nightmare."

Multiple fire department, police and bomb squad vehicles were still parked outside the suspect's house, along a cul-de-sac in southeastern San Jose, hours after the shooting.

An explosives-detecting robot sat in the street near the home while two bomb squad technicians entered the ranch-style house. Arson investigators along with agents of the FBI and US Bureau of Alcohol, Tobacco, Firearms and Explosives were also present.

US President Joe Biden was briefed on the shooting, and his staff continued to monitor the situation while keeping in close contact with local officials to offer any assistance needed, the White House said.

"Our hearts go out to the victims and their families," White House spokeswoman Karine Jean-Pierre told reporters.

Mass gun violence, commonplace in a country with one of the highest rates of firearm ownership in the world, has escalated considerably following a year-long lull as the United States emerged from the worst of the Covid-19 pandemic this spring.

Wednesday's incident was the latest of at least eight deadly US mass shootings in the past three months, including a string of attacks at Atlanta-area day spas in mid-March and a rampage days later that left 10 people dead at a Colorado supermarket.

Last month, a former employee of an Indianapolis FedEx centre shot eight workers to death and then took his own life.