An employee opened fire killing eight people at a commuter train yard in California, police say.
The shooting took place at the Santa Clara Valley Transportation Authority rail yard in San Jose on Wednesday.
Police said bomb squad officers are now checking the scene for explosive devices.
The dead include transit employees and Samuel Cassidy, 57 - named locally as the suspect - at the depot serving Silicon Valley.
San Jose Mayor Sam Liccardo told reporters the mass shooting which broke out at 6.45am local time (2.45pm GMT) was a “horrific day for our city”.
He added that one person died to their injuries while being taken to hospital.
Deputy Russell Davis said the suspect was an employee of the VTA, which provides bus, light rail and other transit services throughout Santa Clara County, the largest county in the Bay Area.
Santa Clara County Sheriff Laurie Smith said it was “undetermined” how the shooter died.
CBS reported that shots broke out during an employee meeting for rail workers.
According to the LA Times, the suspect may have set his two-storey home on fire before embarking on the rampage.
Mr Davis said the attack resulted in “multiple major injuries”.
He did not know the type of weapon used or whether the gunfire happened indoors or outdoors.
Authorities did not immediately release any of the victims’ names.
Santa Clara County District Attorney Jeff Rosen said it was his understanding the shooting happened inside the VTA building.
Victims’ grief-stricken families sat huddled together, holding hands and crying, after learning they had lost a loved one, Rosen told reporters, describing the scene inside a family reunification centre.
Rosen said: “It’s terrible. It’s awful. It’s raw. People are learning they lost their husband, their son, their brother.”
He said about 100 people were inside the building.
Santa Clara County Supervisor Cindy Chavez said of the dead: “These folks were heroes during Covid-19, the buses never stopped running, VTA didn’t stop running.
“They just kept at work, and now we’re really calling on them to be heroes a second time to survive such a terrible, terrible tragedy.”
The massacre shut down traffic on nearby streets during the morning commute.
Light rail services were suspended at noon and replaced with bus bridges, agency chairman Glenn Hendricks told a news conference.
“It’s just very difficult for everyone to be able try to wrap their heads around and understand what has happened,” Mr Hendricks said.
Two patients were transported to Santa Clara Valley Medical Centre, according to spokesperson Joy Alexiou. One person was pronounced dead upon arrival, and the other was in critical condition.
At the scene, Michael Hawkins said he was waiting for his mother Rochelle who had called him from a colleague’s phone to assure him she was safe.
When the shooting started, “she got down with the rest of her co-workers” and dropped her mobile, Mr Hawkins added.
Rochelle Hawkins did not see the shooter, and she was not sure how close she had been to the attacker, her son said.
Santa Clara is the largest county in California’s Bay Area and is home to Silicon Valley - a hub for tech start-ups.
Adobe, PayPal and eBay are all based in the city of about one million residents, which is located about 50 miles southeast of San Francisco.
Across the US, there have been 230 mass shootings so far this year, according to the Gun Violence Archive.
The attack was the county’s second shooting in less than two years.
A gunman killed three people before killing himself at a popular garlic festival in Gilroy in July 2019.
California Governor Gavin Newsom said in a tweet that his office was “in close contact with local law enforcement and monitoring this situation closely.”
Agents from the FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives were responding to the crime scene, officials said.