"SBFD 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.
Police said an active shooter was at the scene in the city about an hour east of Los Angeles, and that they were looking for one to three suspects.
Dozens of people were seen coming out of an area building, which was ringed by heavily armed police.
The man, who did not want his name used, said the area houses office and industrial buildings.
The shooting comes just days after a lone gunman killed three people during a standoff at a Planned Parenthood clinic in Colorado.