Mayor Robert Garcia tweeted that the shooter who opened fire at a law officer there is among the dead.
Long Beach police say the incident is now over but didn't elaborate. They received reports of a shooter around 2:25 p.m. Friday.
Video on local television showed people running from an unmarked building shouting about a shooting inside.
The two-story building is home to several law offices, but police did not specify what kind of business it is.
Dozens of police officers, including members of a SWAT team, surrounded the building.
It is about 20 miles south of downtown Los Angeles in Long Beach, a city of about 460,000 people on the southern tip of LA County.
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