One or more gunmen opened fire inside a building in San Bernardino in California, with reports of 20 victims at a center that provides services for the disabled.
Police were still hunting for the shooter, saying one to three possible suspects were involved.
Heavily armed SWAT teams, firefighters and ambulances swarmed the scene, located about an hour east of Los Angeles, as police warned residents away.
“Area remains VERY ACTIVE. AVOID!” tweeted the local sheriff department, which confirmed there were “multiple victims.”
The city fire department said on Twitter that it was responding to “reports of 20 victim shooting incident” and was working to clear the scene.
Witnesses quoted by local media, said the suspects were seen leaving the scene after the shooting in a black SUV and had left a package behind.
“It appears there is more than one shooter,” Sergeant Vicki Cervantes of the San Bernardino Police told CNN.
She said she could not confirm the SUV report, nor reports that the shooter or shooters were wearing military-style body armor.
CBS Evening News said on Twitter that a bomb squad had been dispatched to the scene to “immobilize what’s believed to be explosive device”.
News reports said the shooting took place at Inland Regional Center, which assists people with developmental disabilities.
Television footage showed dozens of people coming out of a building with their hands raised and walking to a parking lot, ringed by heavily armed police.
One man whose son works in the building where the shooting took place said he received text messages from his son who told him he saw three shooters enter the building.
“He said there were multiple wounded and fatalities,” the man told the local television station KABC.
A spokeswoman for Loma Linda University Medical Center said it was expecting an unknown number of patients from the shooting, according to CNN. She said the extent of the anticipated patients’ injuries is not clear.
The White House said President Barack Obama had been “briefed by his Homeland Security Advisor Lisa Monaco about the shooting and had asked to be updated on the situation as it develops.”
The Democratic presidential hopeful Hillary Clinton tweeted: “I refuse to accept this as normal. We must take action to stop gun violence now.”
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.
Inland Regional Center — the reported site of the shooting — serves thousands of people with developmental disabilities in San Bernardino and Riverside counties, according to its Facebook page.
The center has nearly 670 staff at its locations in the two counties providing services to more than 30,200 people.
The shooting comes just days after a lone gunman killed three people during a standoff at a Planned Parenthood clinic in Colorado.
It was certain to further stoke the bitter about gun control in the United States, where shootings are a part of daily life.