Screencap: KOMO News

Police say that the 19-year-old Washington man who shot and killed three former classmates at a house party in suburban Seattle on Saturday was motivated by jealousy over an ex-girlfriend, one of the victims, KOMO News reports.

According to a probable cause statement, Allen Ivanov bought the AR-15 he used to kill the victims about a week before the shooting. To operate the weapon, Ivanov reportedly sat in his car across the street from the crime scene to read the rifle’s instruction manual before the attack. From CBS News:

Ivanov, 19, told police he had dated [19-year-old Anna] Bui for about a year and a half but broke off the relationship about two months ago because he needed time to work on himself, the documents say. After the breakup, Ivanov said he received “hurtful” Snapchat messages from Bui indicating that she was getting on with her life without him, which made him jealous, the documents said. Ivanov said he decided he wanted to get back together with Bui, the documents say, and the two spent some time together during the week before the shooting. Ivanov said he thought things were “going well” and thought there was a chance they could get back together, but he became angry when he heard from peers that Bui was dating others. The documents also indicate that Ivanov gave a few indications of his troubling intent: He texted someone last week in Tennessee “regarding committing a mass shooting;” he posted on Twitter, “What’s Ruger gonna think” - an apparent reference to the manufacturer of his rifle; and that he told his supervisor at an electronics store on Friday that the previous night he had put the rifle in the trunk of his car and gone to a quiet spot and just sat.

According to Mukilteo Police Detective John Ernst, Ivanov was watching Bui—whom he described as his “dream girl” and first kiss—through an outside window on Saturday when he was discovered by a party attendee. From KIRO:

“The male said, ‘No, no no,’” Ernst wrote. “Ivanov stated that he was ‘scared,’ he flipped the selector switch to fire and shot the male. He stated that at that point it was too late to turn back, and once he had pulled the trigger his adrenaline kicked in.” Ivanov said he entered the house through a side door, found Bui and shot her twice, then continued through the house, saw through the front door another man running toward the house and shot him, according to the probable-cause statement. From a balcony off the master bedroom, he said, he shot at two more men in the driveway before going onto the roof, realizing his magazine was empty and fleeing.

In the end, Bui, 19-year-old Jordan Ebner and 19-year-old Jake Long were dead and 18-year-old Will Kramer was seriously injured. Police say Ivanov confessed to the killings.

Ivanov is currently being held on suspicion of two counts of first-degree murder, one count of aggravated first-degree murder and one count of attempted first-degree murder. If convicted, Ivanov could face the death penalty, KOMO News reports.