LONG BEACH, Calif.—Five gang members were sentenced Jan. 29 to life in prison without the possibility of parole for the shooting deaths of three people at a Halloween-themed party in Long Beach just over four years ago, along with the attempted murders of nine other people who were injured, including a woman who was left paralyzed.

Jurors found David Heng Long, 23, Kaylin Thik, 24, Ryan Sim, 22, Jeremy Penh, 29, and Grant Johnson, 39, guilty last Nov. 15 of first-degree murder for the Oct. 29, 2019, killings of Melvin Williams II, 35, of Gardena, Maurice Poe Jr., 25, of Long Beach, and Ricardo Torres, 28, of Inglewood, who died at the scene of the shooting in the Rose Park area of Long Beach.

The Long Beach jury found true the special circumstance allegations of multiple murders and gang-related murders against all five defendants.

The panel also found true allegations that Mr. Long, Ms. Thik, and Mr. Sim— whom the prosecution alleged each fired shots—personally discharged a handgun during the crime.

In her opening statement, Deputy District Attorney Michele Hanisee told jurors that Mr. Penh mistakenly believed rival gang members would be attending the party that night.

“The plan was to kill rival gang members. The only problem was there were no rival gang members there,” Ms. Hanisee told the jury.

The prosecutor also told jurors that Mr. Penh masterminded the attack after hearing from a childhood friend that a man he believed had disrespected “his hood” in a video was hosting the party, with Mr. Penh then driving Mr. Long, Ms. Thik, and Mr. Sim to the crime scene where the three allegedly opened fire and turned a party of friends and co-workers into a scene of “terror and bloodshed.”

Victims’ Testimony

During the trial, Jasmine Johnson testified that she “knew I was paralyzed immediately” after waiting for the gunfire to stop that night and seeing that the position her feet were in was “completely off.”

Ms. Johnson, who was shot in the neck, said she was hospitalized for two months. She said the shooting left her a paraplegic without any feeling below her chest, adding that she has a “constant fear of loud noises” that is “haunting” and understands that she has a shortened life span and no prognosis to walk in the future.

Ingrid Cortes, who was celebrating her birthday that night, testified that she heard what she thought were fireworks before being struck in the chest.

“I was like, ‘These are not fireworks,’” she said, noting that she saw everyone running when the gunfire erupted.

When asked if the shooting took her by surprise, she said, “It sure did.”

She testified that she eventually blacked out and awoke to her friends trying to take care of her wounds in the home’s living room, and subsequently spent a month in a hospital.

Other partygoers testified that they were shot as they tried to flee when the gunfire rang out.

How Shooting Started

Jurors also heard during the trial from Mr. Sim, who testified that he was solely responsible for the shooting and had fired from three guns.

Mr. Sim told jurors that he and fellow gang members were headed to go bar-hopping in Orange County in three separate vehicles when he asked to be let out of the Toyota 4Runner being driven by Mr. Penh after noticing an alley where he had been jumped and “knocked out” five years earlier.

Mr. Sim—who told jurors that he was about 12 when he became a gang member— testified that he thought “maybe I would run into whoever jumped me in that alley” five years earlier, and said that he asked Ms. Thik to “have my back.”

Mr. Sim said he was armed with a 9 mm gun and a .40-caliber gun, heard sounds from a party, jumped up onto a wall, and thought some of the partygoers may have been from a rival gang.

Mr. Sim said he already had his 9 mm gun out and began shooting as three men approached him and he heard one of them say a word he considered to be associated with the rival gang, but acknowledged during cross-examination that it was a term that others might use.

“Were people screaming?” Ms. Hanisee, the deputy DA, asked Mr. Sim.

He responded affirmatively, saying that he saw people running and falling.

He told jurors that he fired from the 9 mm weapon until it ran out of ammunition and that he jumped down from the wall and “snatched” a gun from the hand of Ms. Thik, whom he said was sprawled face down on the ground in the alley.

“I [saw] the gun so I just take it,” Mr. Sim said, noting that he then began shooting through circular metal rings at the top of the sliding gate to the backyard where the party was being held and fired from his .40-caliber gun that eventually jammed and the gun he had taken from Ms. Thik.

He said he yelled the gang’s name after the shooting and urged Ms. Thik to run faster to get back to a Chrysler PT being driven by Joshua Sam, a former co-defendant who testified during the trial in exchange for what is expected to be a 17-year prison term.

Mr. Sim maintained that there was “absolutely not” any discussion about going to shoot up a party when the group met up that night in north Long Beach as the prosecution alleged.

His testimony contradicted that of Mr. Sam, who was initially charged with the same counts and subsequently agreed to plead no contest to three counts each of voluntary manslaughter and attempted murder in exchange for a lesser prison term. Mr. Sam, now 44, testified that Mr. Penh had told him they would be going to a rival gang’s party to “shut it down.”

Mr. Sam testified that he was instructed to wait nearby to pick up the three gunmen and that he “absolutely” knew there was going to be a shooting.

A forensic firearms examiner concluded that three different guns were used based on a microscopic examination of 23 bullet casings that were recovered from the scene, but no guns were recovered, the prosecutor said.