EVERETT — Snohomish County prosecutors Tuesday charged a University of Washington student with three counts of aggravated murder and one count of attempted first-degree murder for Saturday’s shootings at a Mukilteo house party.

Deputy prosecutor Adam Cornell also added an allegation that the murder of Anna Bui was a domestic violence crime.

Allen Christopher Ivanov and Bui used to date. They had been students together at Kamiak High School and at University of Washington Bothell. Ivanov, 19, allegedly told police that he hunted Bui down at the party because he was jealous and angry that she was dating other guys. He said he’d broken up with Bui about two months ago.

Detectives reported that Ivanov said he wanted to reconcile with Bui, and the pair had spent some time together the week before the shootings. He became angry when he learned that she continued to see other guys during that time, according to court papers.

Ivanov is charged with aggravated murder for the deaths of Bui, Jordan Ebner and Jacob Long, all 19. Prosecutors charged him with attempting to murder Will Kramer, 18. Kramer was shot in the shoulder but is expected to survive the attack. Cornell also charged Ivanov with one count of first-degree assault. He’s accused of pointing his Ruger semiautomatic rifle at the homeowner’s son.

Prosecutors have until Aug. 19 to refile the charges in Superior Court. Ivanov is being held without bail. He made a brief court appearance Monday in Everett District Court. Cornell told the judge that Ivanov faces charges that carry a potential death sentence.

Ivanov was arrested just two hours after police and paramedics were called to a house in the Chennault Beach area. About 20 young people, many Kamiak High School graduates, were inside at the time of the attack.

Ivanov allegedly admitted to the killings and described his motivation behind the ambush, according to court papers. He reportedly told police that he was jealous that his “dream girl” was moving on without him.

The suspect also described how in recent days he’d purchased a Ruger semiautomatic rifle and an extra magazine. The assault-style weapon is similar in appearance to an AR-15. Ivanov told the detective that he’d purchased the rifle about a week before and planned to use it for target shooting and in a gun safety class.

“He admitted that to him the rifle was a symbol of power,” the detective’s report said.

Ivanov allegedly told the detective how he’d driven to the party and spotted Bui with another guy. He returned to his car, read the instruction manual for the rifle, loaded the weapon and crept near the living room windows, according to the report, which was filed in Everett District Court.

The gunfire began about 12:07 a.m., police said.

“Ivanov stated that while he was in his hiding place, he was discovered by a male partygoer,” the report said. “The male said, ‘No, no, no.’ Ivanov stated he was ‘scared,’ he flipped the selector switch to fire and shot the male.”

Ivanov then reportedly said “at that point it was too late to turn back, and once he had pulled the trigger his adrenaline kicked in.”

Detectives found evidence that Ivanov was planning the killings for days, according to court papers. He allegedly sent text messages to someone in Kentucky mentioning a mass shooting, two or three days before Saturday’s attack.

Detectives also recovered tweets they believe Ivanov sent prior to the attack. One referenced the manufacturer of his rifle: “What’s Ruger gonna think?”

Diana Hefley: 425-339-3463; hefley@heraldnet.com.