A criminal trial in one of the worst mass shootings in the city’s history took a turn Tuesday when a 22-year-old gang member took the stand and said that he acted alone in killing three people at a Halloween party four years ago.

Ryan Sim, who was 18 at the time of the crime, testified that he shot up the Halloween-themed birthday party that killed three people and wounded nine others on the night of Oct. 29, 2019.

He said it was a chance to catch what he believed to be rival gang members “slipping” after being “jumped” by them in the same area five years earlier.

He said the rest of his co-defendants —Jeremy Penh, 29; David Heng Long, 23; Kaylin Thik, 24; and Grant Johnson, 39 — had no prior knowledge that there would be a shooting.

This contradicts the core contention of prosecutors, who have charged them all with nine counts of attempted murder and three counts of murder for the deaths of 35-year-old Melvin Williams II, 28-year-old Ricardo Torres and 25-year-old Maurice Poe Jr., who were attending the party that night.

Prosecutors previously presented evidence painting Penh was the “shot caller” and mastermind behind the shooting, believing mistakenly that there would be rival gang members in attendance.

Using testimony from witnesses and evidence such as surreptitiously recorded statements from the jailed defendants, authorities allege Penh handpicked Sim, Long, and Thik to be the shooters as a way for them to “earn their stripes.” Four others — Joshua Sam, Johnson, Danny Sourn and Christopher Williams —would serve as either getaway drivers or decoys, according to prosecutors.

That seemed to be the narrative of events, with Sam, a now-former Tiny Rascals Gang, testifying last week in court that he served as a getaway driver the night of the shooting, pointing at Penh as the person behind the plan, and Sim, Long, and Thik as the shooters.

But on Tuesday Sim decided to testify on his own behalf, claiming he used three different guns in a span of five minutes to shoot up the party.

Sim, explaining his version of the events, said he was at the beach the day of the shooting playing basketball with Thik. Sim, because of his age, called Penh asking if he could buy him a bottle of alcohol.

Penh eventually picked the two up from the beach and took them to buy the alcohol before heading to the Brayton Avenue apartments in North Long Beach where the gang was known to frequent.

There, according to Sim, they agreed to head out to Orange County to find some parties or bars they could stop by. First, they stopped at an Arco gas station where Sim said he gave Sam a few dollars to put in his car. The crew then made its way 7 miles down Orange Avenue with the intention to eventually turn left to head toward the freeway and reach Orange County, Sim said.

When they got to the area of Ohio Avenue, however, Sim said he got “deja vu” and remembered he had been jumped in the area, when he was 13, by a group of rival gang members and he thought maybe he could “catch them slipping” five years later.

With two guns, Sim asked Penh to let him out of the car in the nearby alleyway and that he’d call him if he needed anything. Sim then asked Thik to accompany him in case he happened to run into any rival gang members.

Within a few minutes of walking in the area, Sim said he noticed the party was going on. That’s when he got closer to the area and “squatted like Batman” onto the brick fence.

He said during testimony that he wasn’t sure if they were rival gang members at the time, only seeing people walking around in masks and costumes. At some point, Sim said three men noticed him and walked in his direction.

He testified that he heard one of them say “cuh,” and that’s when he emptied his clip in their direction.

When he ran out of bullets from his first gun, he hopped down from the fence, and snatched a gun away from Thik, who was lying down flat on the floor and not participating in the shooting, he said. Sim said that he then got both guns and placed them through openings in a sliding fence and kept firing into the party as people ran and screamed.

“I was just aiming at anybody,” Sim said. They then noticed Sam’s PT Cruiser was at the end of the alley and they ran into his car, Sim said. Sam then drove them back to the Brayton Avenue apartments where Sim testified to stashing the guns in the garden before heading to the beach.

Sim, Long, Penh and Thik went to Fresno the following day and bounced around the “homies'” houses for a while, the 22-year-old said. Sim testified that he was eventually taken into custody in connection with the shooting while in San Jose sometime in September 2020.

While in jail, Sim was recorded bragging about shooting up the party to undercover agents in what’s known as a Perkins Operation, describing that he felt like he was “in the movies” while pulling the trigger. He later backtracked on those comments while testifying and said he wanted to look good in front of the Perkins Operation agents he thought were inmates because they appeared to be part of a more influential gang in jail and he was scared of them.

During cross-examination, Deputy District Attorney Michele Hanisee questioned Sim’s recollection of the events, along with the crew’s decision to travel 7 miles down Orange Avenue when they were allegedly headed to Orange County, especially since the 405 Freeway was so close to the gang’s hangout on Brayton Avenue.

Penh’s attorney, meanwhile, has not denied that her client was a member of Tiny Rascals Gang, but has maintained that he didn’t plan the attack and is not guilty of any crimes because he wasn’t actually present at the time of the shooting. Long’s attorney has also argued that his client was never actually seen at the scene of the crime with a gun in his hand by anyone testifying.