Los Angeles - Police were hunting Wednesday for one or more gunmen who opened fire inside a building in San Bernardino in California, with reports of 20 victims.
Heavily armed SWAT teams, firefighters and ambulances swarmed the scene, located about an hour east of Los Angeles, as police warned residents away.
"San Bernardino Fire Department units responding to reports of 20 victim shooting incident in 1300 block of S. Waterman. SBPD is working to clear the scene," the city fire department said on Twitter.
"Area remains VERY ACTIVE. AVOID!" tweeted the local sheriff department, which confirmed there were "multiple victims".
Police said that one to three possible suspects were involved.
CBS evening news reported that the shooting took place at Inland Regional Centre, which assists people with developmental disabilities.
It said on Twitter that a bomb squad had been dispatched to the scene to "immobilise what's believed to be explosive device".
Dozens of people with their hands raised were seen coming out of an area building and walking to a parking lot, ringed by heavily armed police.
Loma Linda University Medical Centre was expecting an unknown number of patients from the shooting, spokesperson Briana Pastorino said. She said the extent of the anticipated patients' injuries is not clear.
The shooting took place near a golf course where an employee told AFP that staff had been told to stop all activities.
The man, who did not want his name used, said the area houses office and industrial buildings. He said there were also government offices in the area.
The shooting comes just days after a lone gunman killed three people during a standoff at a Planned Parenthood clinic in Colorado.