The suspects were members of a gang who incorrectly believed they were opening fire on rival gang members, authorities said, even though no one attending party was a member of a gang.By CBSLA Staff

LONG BEACH (CBSLA) – Twelve people have been arrested in connection with a mass shooting last October during a Long Beach Halloween party which left three men dead and nine other people injured.

FILE — An aerial view of a Long Beach home on Oct. 29, 2019, where three men were killed and nine others wounded when several gunmen opened fire. (CBSLA)

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In a news conference Tuesday morning, Long Beach Police Chief Robert Luna disclosed that the suspects were members of a gang who incorrectly believed they were opening fire on rival gang members, even though no one attending the party was a member of a gang.

“It was a preplanned attack in what the suspect incorrectly believed to be a party of rival gang members,” Luna said.

According to Luna, 10 of the 12 suspects were arrested during a multi-day operation across California that saw several law enforcement agencies serve 15 search warrants. The other two suspects were already in custody on unrelated charges of murder and attempted murder. No names were immediately released.

Seven of those warrants were served in Long Beach, and eight in various other cities. Eight firearms were seized, including three ghost guns.

The warrants were part of the investigation into the shooting which occurred on the night of Oct. 29, 2019, at a home in the 2700 block of 7th Street.

Back in July, police had released surveillance video of three SUVs – a Toyota 4Runner, Toyota Rav4 and a Silver Chrysler PT Cruiser — which investigators believed carried the suspects from North Long Beach to the 7th Street home on the night of the killings.

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The suspects opened fire from an alley and into the backyard of the home where a crowd of 25 to 30 people were celebrating. Three men were killed and nine others –including seven women and two men – were wounded.

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Eight suspects actually drove to the alley, but only three opened fire, Luna said. Ballistics confirmed that shots were fired from three different handguns.

“Eight cowardly suspects who are all from a local Long Beach gang, parked in the alley, walked towards the fence to shoot into the backyard,” Luna said.

Luna described the case Tuesday as “extremely complex and detailed.” The three suspected shooters are among those under arrest.

“We believe that we have the three shooters that were responsible for firing indiscriminately into this backyard…We anticipate that the Los Angeles County District Attorney’s Office will charge them all with murder,” he said.

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Luna also added that the majority of the remaining nine suspects will likely also be charged with murder, while a few could face gun charges.