Police arrived to find the gunman and one victim dead and learned that the second victim had driven himself to a hospital, Long Beach police Sgt. Brad Johnson said at a news conference.
The injured man was in stable condition.
Police didn’t fire any shots, Johnson said. A SWAT team searched the rest of the building and no other victims were found.
The gunman’s motive had not yet been determined, but authorities emphasised it was not a case of an active shooter targeting as many people as possible.
“This incident was workplace violence,” Johnson said.
Video showed people running from a two-story office building shouting about a shooting inside.
The building in a well-to-do neighborhood in Long Beach is home to three law firms, but police did not reveal the office where the shooting occurred.
The site is about 20 miles south of downtown Los Angeles in Long Beach, a city of about 460,000 people.