The shooting occurred about 10:40 p.m. last Oct. 29 at a Halloween-themed party in the 2700 block of East 7th Street in the Rose Park area.
Maurice Poe Jr., 25, of Long Beach; Melvin Williams II, 35, of Gardena; and Ricardo Torres, 28, of Inglewood, died at the scene.
Long Beach police Chief Robert Luna said authorities from various law enforcement jurisdictions served 15 search warrants, seven in Long Beach and eight in various other cities, and made arrests from Sept. 2-5. He said two of the 12 suspects were already in custody on unrelated crimes.
About 25-30 people were in the backyard of the home when suspects opened fire into the crowd from the alley, the chief said.
"Ballistics confirmed that the shots were fired from three different handguns,'' Luna said during a morning briefing on the case.
He said investigators determined that eight suspects, who are in a Long Beach gang, traveled in three vehicles to the crime scene and targeted the gathering. The eight suspects parked near the home and walked to the alley, Luna said.
"This is an extremely complex and detailed case, and we now know that this was a pre-planned attack on what the suspects incorrectly believed to be a party of rival gang members,'' Luna said. "None of the victims at that party were gang members.''
Many of the partygoers worked together and lived outside of Long Beach, he said.
Luna said authorities seized eight firearms during the course of the investigation, and forensic analysis was pending to determine if they were used in the shooting.
"We believe that we have the shooters -- the three shooters that were responsible for firing indiscriminately into this backyard -- in custody,'' Luna said. "We anticipate that the Los Angeles County District Attorney's Office will charge them all with murder.''
Luna said he believes the majority of the suspects will face murder charges and the others will face weapons charges.
Police identified those arrested in the three-day period last week as:
-- David Long, 20, of Long Beach, arrested in Long Beach and booked on suspicion of murder, with bail set at $2 million;
-- Christopher Williams, 28, of Redlands, who was arrested in Redlands, booked on suspicion of murder and is being held on $2 million bail;
-- Ryan Sim, 18, of Long Beach, was arrested in San Jose, booked on suspicion of murder and is being held on $2 million bail;
-- Kaylin Thik, 21, of Ontario, was arrested in Ontario, booked on suspicion of murder and is being held on $2 million bail;
-- Derrik Mith, 27, of Ontario, was arrested in Ontario, booked on suspicion of being a felon in possession of a firearm and was held on $35,000 bail;
-- Joshua Sam, 41, of Long Beach, was arrested in Long Beach, booked on suspicion of murder and is being held on $2 million bail;
-- Grant Johnson, 35, of Orange, was arrested in Orange, booked on suspicion of murder and is being held on $2 million bail;
-- Dara Chhun, 27, of Spring Valley, was arrested in Spring Valley, booked on suspicion of being an accessory to murder and is being held on $1 million bail;
-- Samnang Long, 37, of Long Beach, was arrested in Long Beach, booked on suspicion of possession of a firearm and was held on $35,000 bail; and
-- Andy Prok, 28, of Long Beach, was arrested in Long Beach, booked on suspicion of carrying a loaded firearm in public and was held on $35,000 bail;
Two other suspects were already in custody on unrelated crimes, detectives believe they are involved in the 2019 killings, police said. They are:
-- Danny Sourn, 26, of Long Beach, who was already in custody in connection with a 2018 killing police said. Sourn was booked on suspicion of murder, and is being held on $2 million bail; and
-- Jeremy Penh, 25, of Long Beach, who was already in custody in connection with an unrelated attempted murder in 2020. Penh is being held on $1 million bail.
Following the killings, the man who rents the home where the shooting occurred told the Long Beach Post that his son, Daniel Chan, had been hosting a birthday party for one of his co-workers. Chan Hou said his family was safe, but some friends were killed.
Poe was an aspiring entrepreneur with a 3-year-old daughter and "was full of life,'' his father told the Post.
Williams sang in his church choir and was "the light that brightened up the room,'' his stepbrother told the Long Beach Press-Telegram.
Torres was a line cook at the unnamed South Bay company where many of the partygoers worked.
"He was a great person ... he always made you feel welcomed,'' a friend told the Post.
Police identified the wounded victims as a 20-year-old woman from Anaheim, a 27-year-old woman from Hawthorne, a 27-year-old man from Cudahy, a 28-year-old woman from Los Angeles, a 28-year-old woman from Gardena, a 29-year-old man from Hawthorne, a 30-year-old woman from Compton, a 32-year-old woman from Lawndale and a 49-year-old woman from Los Angeles.
Long Beach mass shooting: 3 dead, 9 injured after gunfire erupts at party