The San Bernardino Fire Department tweeted that it was responding to a "20 victim shooting incident" and it was working to clear the scene.
It is still a "very active scene" and police are trying to secure the building, said a spokeswoman from the San Bernardino Police Department.
There may be up to three gunmen she said, and the shooters were heavily armed and possibly wearing body armour.
She did not confirm the number of victims.
A local reporter tweeted that people are being evacuated from the Inland Regional Center, a non-profit medical and health organisation.
Doesn't sound good.