Investigators were trying to determine whether the shooting was gang related, said Los Angeles County sheriff's Deputy Keith Ho.
The shooting occurred Saturday evening during a fundraiser held by a motorcycle group called the Old School Riders. People gathered in the parking lot of Falcone's Pizza were fired upon by a gunman who then jumped into a dark-colored vehicle, Ho said.
"The suspect started firing into the crowd without provocation," the deputy said.
Three men, two of them brothers, died at the scene. Seven others were hospitalized in conditions ranging from moderate to serious, Ho said.
Nobody inside the restaurant was injured.
The Old School Riders Web site said security would be provided for the event.
The group's mission is "to enjoy the outdoors and also join together to participate in charitable events." The Web site also said the group is made up of family and friends and no one represents any specific biker club.
Pico Rivera is about 15 miles east of downtown Los Angeles.
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