Fire officials in San Bernardino said it was responding to a “20 victim shooting incident” at the Inland Regional Center and it was working to clear the scene.
ABC News reports that there are 12 people dead but police have not confirmed that number.
It is still a “very active scene” and police were trying to secure the building, said a police spokeswoman.
There may be up to three gunmen, she said, and they were heavily armed and possibly wearing body armor.
The gunmen may have fled the scene in a black SUV and may have had long guns.
One man received a text from his daughter that said she was hiding inside the building, where she works.
Terry Petit told reporters his daughter wrote to him: “People shot. In the office waiting for cops. Pray for us. I am locked in an office.”
President Barack Obama has responded to the shooting.
“One thing we do know is that we have a pattern of mass shootings in this country that has no parallel anywhere in the world,” he said.
“There are some steps that we could take, not to eliminate all incidents, but to make sure they happen less frequently.”
A local reporter tweeted that people were being evacuated from the Inland Regional Center, a medical and health organization.
The organization administers help to people with disabilities like autism and other mental health problems.
The shooting is happening less than a week after a shooting at a Planned Parenthood clinic in Colorado left three people dead and nine people wounded.
People are being seen wheeled away on stretchers with medics and police officers in attendance.
CNN reports that area hospitals are receiving at least 15 patients from the shooting.
An employee of the centre told CNN that the building was holding an event for county personnel inside an auditorium when the shooting occurred.
Representatives from the FBI and the Federal Bureau of Alcohol, Tobacco and Firearms are on the scene.
No arrests have been made yet.
There are also unconfirmed reports of an explosive device at the scene.
Witnesses told CNN they heard rapid shooting coming from nearby the building.
Police confirmed a report that the suspects entered a conference room in the building and started shooting but had no further details.
There are three buildings on the campus with more than 600 employees total, said Kristin Myricks, an employee of the centre who did not go to work today.
The facility has had active gunman training in the past. Badges are needed to enter the facility.
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