An assailant opened fire at a light rail yard in San Jose, California, on Wednesday morning, killing an unspecified number of people and injuring others, the county sheriff's office said.
Santa Clara County Sheriff's Deputy Russell Davis told reporters that "there are multiple injuries and multiple fatalities" and the assailant was dead.
"I can't confirm the exact number of injuries and fatalities," he said. He said he could not identify the shooter or confirm how they died.
The shooting took place just before 7 a.m. Pacific Time (1400 GMT) at a light rail yard run by the Santa Clara Valley Transportation Authority near the city's main airport.
"The shooter is no longer a threat, and the facility has been evacuated," San Jose Mayor Sam Liccardo wrote on Twitter.
San Jose, a city with about 1 million residents, lies at the heart of Silicon Valley, a global center of technological innovation and home of some of America's biggest tech companies.
Mass shootings are common in the United States, which has one of the highest rates of gun ownership in the world.
In March, eight people, including six women of Asian descent, were shot dead in a string of attacks at day spas in and around Atlanta, and less than a week later, 10 people were killed at a supermarket in Boulder, Colorado.