The dead included one victim and the suspected shooter, police said.
The shooting was reported at a Long Beach law office about 2:25 p.m. Local time (2225 GMT) and police rushed to the scene immediately.
Police confirmed later that there was no longer an active-shooter threat at the scene, Xinhua news agency reported.
A video on Twitter showed there was a heavy police presence at the location. The area was cordoned off by police.
Long Beach City Mayor Robert Garcia said on his Twitter page that the incident "appears to be a workplace homicide."
"Both the shooter and a victim are dead. One other person was shot but is in hospital in stable condition. Our thoughts are with the victims and their families," he tweeted.
All three involved -- the shooter and both victims -- were employees at a law firm in the 300 block of E. San Antonio Drive, reported the Press-Telegram, a daily newspaper published in Long Beach.
The shooter didn't engage with police officers and died from a self-inflicted wound, Sgt. Brad Johnson was quoted as saying by the Press-Telegram.
Investigation into the shooting was ongoing, police said.
Long Beach is a port city within the Greater Los Angeles area of Southern California.