Police say 19-year-old Allen Ivanov, shot and killed three people at a house party over the weekend because was jealous over losing his girlfriend.

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Ivanov had been dating 19-year-old Anna Bui, who he described as his “dream girl,” for over a year before they broke up, with Ivanov saying that he needed time to work on himself.

Although the two of them still hung out, and Ivanov said that things were “going well,” he became upset when he heard from friends that she was dating other people, sending him in a rage.

He tweeted warnings that he would do something with his AR-15 rifle, posting pictures of the gun and bullets on Instagram.

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Mukilteo Police Detective John Ernst said that Ivanov was watching Bui on Saturday through a window during a house party before the shooting. An attendee approached him and confrontation began.

“The male said, ‘No, no no,’” Detective Ernst wrote. “Ivanov stated that he was ‘scared,’ he flipped the selector switch to fire and shot the male. [Ivanov] stated that at that point it was too late to turn back, and once he had pulled the trigger his adrenaline kicked in.”

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Witnesses say Ivanov then went inside the party and shot Anna Bui to death. He also shot and killed 19-year-olds Jordan Ebner, Jake Long, and seriously injured 18-year-old Will Kramer.

Ivanov, confessed to the killings and is 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.