Allen C. Ivanov was arrested by state troopers on Interstate 5 early Saturday more than 100 miles from the bloody scene in Mukilteo, a north Seattle suburb, authorities said.

They said he confessed to the killings and that he did it because he was angry that his ex-girlfriend, Anna Bui, seemed to be moving on with her life after their recent breakup. She was one of the victims.

The document also indicates 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 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.

Ivanov made a court appearance by video link Monday from the Snohomish County Jail and a judge ordered him held without bail, said Dave Wold, a spokesman for the prosecutor's office. Prosecutors have until Wednesday to file charges.

Ivanov's attorney, Tim Leary, did not contest the state's request that his client be held without bail.

Ivanov had bought the assault-style weapon about a week before the attack, saying he planned to use it for target practice and that he had signed up for a gun-safety class this month, Mukilteo Police Detective John Ernst wrote in the probable-cause statement.

Police haven't said where he bought it, but Ernst wrote that Ivanov said he went to a sporting goods store on Friday to buy a second magazine for it.

"Ivanov stated that he showed up to the homicide scene at approximately 2200 hours, and parked across the street and watched," Ernst wrote. "He said that he creeped up toward the house and saw A with another male and got angry. He said that he returned to his car, read the instruction manual for the rifle, loaded the magazine, placed the magazine in the rifle, and sent the rifle's selector switch to 'safe.' He then returned to the victim house property."

Ivanov told detectives he creeped around the back of the house and hid near the living room windows, where he was eventually discovered by one of the young men attending the party.

"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.

Ivanov's attorney questioned why it's legal for a depressed 19-year-old to buy a semi-automatic rifle, especially given research that shows the brains of young adults continue to develop into their 20s.

"If he would have walked into a 7-Eleven and tried to buy a six-pack of beer, he would have been turned away," Leary said. "When you look at someone who's 19, what they can and can't do is very troubling, and the consequences of that could not be any greater for the victims, for my client and for the community at large."

In addition to Bui, of Everett, Jordan Ebner, of Lake Stevens, and Jacob Long, of Everett, were killed. They were all 19 and recent graduates of Kamiak High School in Mukilteo, a waterfront city of 21,000 people.

A fourth person, 18-year-old Will Kramer, was wounded and remained in serious condition Monday at Harborview Medical Center in Seattle.

Bui and Ivanov were students at the University of Washington.

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