The Latest on a shooting at a small party in Mukilteo, Washington, that left three people dead and one injured early Saturday (all times local):

10:55 a.m.

Court documents say a 19-year-old man who shot and killed three people at a party in Washington state was so unfamiliar with his newly purchased AR-15 semi-automatic rifle that he parked his car across the street from the party and read the firearm's instruction manual just before the attack.

Authorities say Allen C. Ivanov was arrested by state troopers on Interstate 5 about 100 miles from the bloody scene he left in Mukilteo, a well-off Seattle suburb. In a probable cause statement released Monday, they say he confessed to the killings and that he did it because he was angry that his ex-girlfriend seemed to be moving on with her life after their recent break-up. She was one of the victims.

The document also says Ivanov had texted someone in Tennessee about his plans a couple of days before the shooting. It's unclear whether the recipient of those texts tried to report them to anyone.

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8:47 a.m.

A 19-year-old man suspected of fatally shooting three people at a party in a Seattle suburb over the weekend is expected in court Monday afternoon.

Allen Ivanov is being held in the Snohomish County Jail.

The Snohomish County Medical Examiner's Office has identified the three 19-year-olds who were killed at the party early Saturday in Mukilteo as Jordan Ebner, of Lake Stevens; Jacob Long, of Everett; and Anna Bui, of Everett. They died of gunshot wounds. A fourth person was also seriously injured.

Hundreds attended a vigil Sunday at a local church to remember the victims. The Daily Herald of Everett reports they were recent graduates of Kamiak High School in Mukilteo, a waterfront city of 21,000 north of Seattle.

Both Bui and Ivanov were students at the University of Washington.