An employee shot dead at least eight people at a rail yard in California before taking his own life, police said, in the latest mass shooting to hit the US.
"I know for sure that when the suspect knew that law enforcement was there he took his own life. Our deputies were right there at that time," Santa Clara County Sheriff Laurie Smith told a press conference.
Several others suffered major injuries in the incident at the public transit maintenance yard in San Jose, just south of San Francisco.
Bomb squads were deployed after reports of explosive devices within the compound.
They are trying to "clear out every room and every crevice" of the building, said Russell Davis, a Santa Clara County Sheriff's deputy.
"There are eight victims that are pronounced deceased at this point and there is one suspect that's pronounced deceased," said Mr Davis, adding those numbers could rise.
"The suspect is a VTA employee," he added, referring to the local Valley Transportation Authority.
Police had rushed to investigate multiple early morning 911 calls reporting gunshots.
No immediate details were provided on whether the gunman was killed by police or took his own life, or about the type of weapon he used.
"Our hearts go out to the victims and their families," White House deputy press secretary Karine Jean-Pierre told reporters.
"What's clear, as the president has said, is that we are suffering from an epidemic of gun violence in this country," she added, renewing calls for Congress to pass long-awaited gun control reforms.
Dozens of patrol cars and fire engines as well as FBI officials lined the streets near the rail yard in San Jose, a Silicon Valley tech hub of almost a million people.
Local officials said the shooting, which was initially reported before 7am local time (3pm Irish time), had taken place at a union meeting, with at least 80 staff on site at the time.
Several people were receiving medical treatment, said San Jose Mayor Sam Liccardo.
"Our hearts are pained for the families of those we have lost in this horrific shooting," he tweeted.
The incident took place in a VTA yard that is used to store and maintain trains.
"Our thoughts and love go out to the VTA family, the organization and what they have had to go through," VTA board of directors chairman Glenn Hendricks told reporters.
"I could not be more proud of the VTA organization. This is a horrible tragedy that occurred."
The VTA tweeted that its employees had been evacuated.
San Jose, a city with about one million residents, lies at the heart of Silicon Valley, a global centre of technological innovation and home of some of America's biggest tech companies.
"These are, and were, essential workers," San Jose Mayor Sam Liccardo said at a news conference.
"These VTA employees helped us get through this horrific pandemic.
"They were showing up every day 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 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 told reporters.
Mass shootings are common in the United States, which has one of the highest rates of gun ownership in the world.
A spate of incidents have unfolded as the country has started to emerge from the worst of the Covid-19 pandemic.
In March, eight people, including six women of Asian descent, were shot dead in a string of attacks at spas in and around Atlanta, and less than a week later, 10 people were killed at a supermarket in Boulder, Colorado.
