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Los Angeles Sheriff's detectives launched an investigation into the homicide possibly related to a motorcycle club, according to deputies of the Sheriff's Headquarters Bureau.
Three men, two of whom were brothers, died at the scene, Deputy Byron Ward said.
Seven other people were wounded and taken to hospitals, and one of them died later, he said.
The shooting was reported outside a Pizza restaurant at Slauson Avenue Saturday evening, Ward said.
A motorcycle club was throwing a party at the pizza place, according to the Los Angeles Times.
Richard Hayes, 48, told the newspaper he was heading to a party held by the Old School motorcycle club at the restaurant when he heard about a dozen shots while parking his car.
He said he thought the shooting victims may have belonged to the club.
According to witnesses, a man fired a handgun into the crowd without provocation, striking several of the attendees, then entered the passenger side of a dark-colored vehicle, which sped away.
Apparently, no immediate arrests were made.
The motive for the killing was still under investigation, authorities said.