Boulder Police arrested a man Monday morning on suspicion of first-degree murder in the fatal shooting of a 19-year-old University of Colorado Boulder student on Halloween.

Omar Perez, 18, was booked in the Boulder County Jail at 9:39 a.m. Monday and faces one count of first-degree murder, one count of attempted first-degree murder and one count of first-degree assault causing serious bodily injury with extreme indifference, according to the jail log. Perez, of Thornton, was arrested early Monday with help from the Thornton and Westminster police departments and the Adams County Sheriff’s Office, according to a city of Boulder news release.

If convicted, a first-degree murder charge carries a life sentence in prison.

“As the criminal prosecution gets underway, I promise you that our team will be relentless in the pursuit of justice,” Boulder County District Attorney Michael Dougherty wrote in a statement.

Police believe Perez is the only shooter related to the Marine Street killing, the release says. He is not a CU Boulder student.

Witnesses told police they watched a man pull out a gun and fire three shots indiscriminately into the group of people hanging out at the party, according to an arrest affidavit. Police later identified the man as Perez after someone showed officers Perez’s tracking data for that night and said he recently bought a Glock “on the street.”

One man told police that Perez and a group of people tried to get into the house in the 700 block of Marine Street despite being told they couldn’t, the affidavit says. Most of those people were pushed out of the house, but Perez was still inside and ran at partygoers, the man told police.

The man told police he heard Perez say, “Oh, I could shoot this place up,” after being told to leave, the affidavit says.

The man put Perez in a headlock and started to drag him toward the door around 11:28 p.m. when Perez pulled out a gun and shot three times, the man told police. Another witness told police they saw the same thing, according to the affidavit.

That’s when Perez, who wore a SWAT uniform and a mask from the movie “The Purge,” ran out of the house, the affidavit says. The man told police he went over to a man who was shot in the neck and told him it would be OK.

The person who was shot in the neck, a 19-year-old male CU student, later died at Boulder Community Health, police said. The other 19-year-old student was shot in the leg and is still in the hospital, according to the Boulder news release.

The two 19-year-old students were taken to Boulder Community Health, where one later died, Police Chief Steve Redfearn said in a Sunday news conference.

Police identified Perez after someone called the police tipline and said Perez and three friends came to where she worked in Broomfield, where she saw him wearing the same outfit witnesses at the party described, the affidavit says. Perez told her that he and his friends were beaten up at a party earlier in the night, she told police.

She also told police that Perez bought a 10-millimeter Glock “on the street” recently and posted it on Instagram. In September, Perez shot himself in the calf with a revolver he owned, but dumped it in bushes in Adams County before deputies found it, she told police.

The Instagram post was later deleted, but police found several 10-millimeter bullet casings in the area of the Boulder shooting, the affidavit says.

Perez’s cellphone tracking information showed he was at the Marine Street party from about 11 p.m. to 11:30 p.m. on Saturday night, she showed police.

Police were at the same house around 10 p.m. to shut the party down for a noise violation, the affidavit says. One resident told police as many as 800 people were there at the party’s peak. Most of them left when police shut it down, but a smaller group stayed, the affidavit says.

“Everyone in our community deserves to feel safe and while I hope this arrest restores some measure of safety, I know it will never bring back the life of this young man or undo all the trauma victims have gone through,” Boulder Police Chief Steve Redfearn wrote in the release.

Police are still investigating and ask that people help identify other people who were at the party, the release says. Photos of those people can be found on the city of Boulder website. Anyone who recognizes the people in the photos should call the police at 303-441-3333 and reference case 26-10148, the release says.

The killing was Boulder’s first homicide of the year, Redfearn said Sunday.

“Our students and families deserve to feel safe in their communities, on campus, and wherever they gather,” Gov. Jared Polis wrote in an X post Monday morning. “My thoughts are with the victims, their families, and the entire CU Boulder community.”

Perez is scheduled to appear for a hearing on advisement Tuesday afternoon in Boulder County District Court. Bail has not been set.

This is a developing story and may be updated.