MUKILTEO — A Mukilteo teen suspected of killing three young people at a party here Saturday allegedly told police that the violence was prompted by jealous rage over his ex-girlfriend moving on without him.

Detectives also have found indications Allen Christopher Ivanov, 19, was planning the killings for days, according to court papers released Monday.

Among other things, they’ve spoken with a witness from Kentucky. That person was allegedly sent text messages from the suspect about a mass shooting, two to three days before Saturday’s attack. A detective verified the existence of the texts, court papers said.

Ivanov reportedly agreed to speak with investigators after he was arrested within hours of the shootings.

“Ivanov stated everything that went on tonight was about a girl,” Mukilteo police detective John Ernst wrote in a preliminary report. “He said that he had dated the female homicide victim… for about a year and a half. He stated that she was his ‘dream girl.’ He claimed that she was the first girl he ever kissed.”

Ivanov allegedly said that he’d broken off the relationship a couple of months earlier but became jealous that Anna Bui, 19, was starting to see other guys. They had been students together at Kamiak High School and at University of Washington Bothell.

Ivanov told police he wanted to reconcile with Bui, and in the past week they had spent some time together. The teen’s anger resurfaced when he learned that Bui continued to see other guys, Ernst wrote.

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 Chennault Beach area home where Bui was attending a party early Saturday. He said he became angry when he spotted her with another guy.

That’s when 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.”

He said he went inside, hunted Bui down and shot her twice. At least four spent shell casings were found near the young woman’s body. Ivanov then described shooting other people until the rifle’s magazine was empty. He then left and was captured during a 2:03 a.m. traffic stop more than 100 miles away near Chehalis in Lewis County.

Ivanov later told detectives that when he first reached his car he began to load a second magazine with the intention of returning to the party.

Witnesses to the shootings provided Ivanov’s name to police. Even as detectives were headed to the scene, a police dispatcher already had looked up the suspect on social media and found an image of him posing with a semiautomatic rifle, police said.

Investigators also pulled up a Twitter account that apparently was used by Ivanov. There were two tweets posted Thursday. One read: “What’s Ruger gonna think?” The other said, “First and last tweet. I’ve been through it all.”

Ivanov said that on that same day he had placed the rifle in the trunk of his car and found a quiet spot where he “just sat.”

The day before the shootings, Ivanov said he left his job early at the Alderwood mall, telling his boss he didn’t feel well. He then went to Cabela’s in Marysville to buy the second magazine for his rifle.

Court papers describe in detail the carnage that greeted police and medics who raced to the home after the gunfire. The victims’ bodies were visible inside and outside the home. Up to 20 young people had been at the party when the gunfire began.

In addition to Bui, the dead were Jordan M.R. Ebner and Jacob M. Long, both 19. The killings were the first homicides in Mukilteo since 2002.

Will Kramer, 18, also was shot. He was being treated in intensive care at a Seattle hospital. Kramer is a student at UW in Seattle.

Saturday’s shooting was the worst outbreak of violence in Snohomish County since Oct. 24, 2014. That’s when a 15-year-old freshman opened fire inside a Marysville Pilchuck High School cafeteria, killing four friends, before he turned the gun on himself. The investigation revealed that the boy was upset and suicidal over the recent break-up with his girlfriend. The girl attended a different school and was not injured.

Ivanov appeared briefly before Judge Anthony Howard on Monday afternoon. The judge found probable cause to hold him without bail for investigation of aggravated murder, two counts of first-degree murder and one count of attempted first-degree murder. Howard also ordered that Ivanov not contact the victims’ families.

Detectives spoke with them and they don’t want the defendant released, Snohomish County deputy prosecutor Adam Cornell told the judge. In asking that Ivanov not be permitted to bail out, Cornell said the suspect is accused of aggravated murder — a crime that carries a potential death sentence. Prosecutors have until midweek to file charges.

Ivanov is represented by Seattle attorneys Tim Leary and Zachary Wagnild. The lawyers didn’t argue to reduce bail.

Leary questioned why it’s legal for a depressed 19-year-old to buy a semiautomatic 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.”

A group of young men attended Monday’s hearing. They said they were classmates of the deceased. Sultan Akbar, 18, of Mukilteo, said he also was friends with Ivanov in high school. He was stunned to learn that his former “good buddy” had been arrested for the killings.

“It’s crazy. A mass murderer? I’m completely shocked,” Akbar said.

The court hearing occurred around the same time as a vigil at UW’s Bothell campus in memory of Bui, who had been a student there. Ivanov also studied at UW Bothell.

At Everett Community College, counseling was being offered Monday to students and staff. Ebner, of Lake Stevens, had been a student there.

“These are tough times and the stress on both the college and the community will require us to be sensitive to each other — especially to those who knew the young people involved,” said college president David Beyer in a message Monday.

Mukilteo police continue to ask anyone with information about the shootings to call their tip line at 425-263-8108.

The Associated Press contributed to this report.