The local fire department confirmed three fatalities and said nine patients were transported to local hospitals.
Witnesses told the Los Angeles Times there might have been a Halloween party at the house where the shooting began.
Some told the paper they heard up to 20 shots.
People in costumes could be seen limping out of the building, another witness told the Long Beach Press Telegram.
The Long Beach Fire Department said they were called to a shooting at the home around 10.45pm on Tuesday night.
Jake Heflin, a spokesperson, said they arrived at a scene “full of chaos”. He said the victims appeared to be in their 20s.
The suspect or suspects are still at large, he added.
One witness, Martin, told the Long Beach Post he was walking nearby when he heard a car swerving and accelerating ahead of him.
“Then all I hear is pop, pop, pop, a bunch of pops– maybe like 20 shots,” he said. “It was a lot.”
He then ducked and ran in the opposite direction.
The Long Beach Police Department said it was investigating the shooting.
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