A gunman attacked a gathering of young adults at a suburban Seattle home early Saturday, killing two people at a fire pit before firing more shots from the roof, the grandmother of one of the witnesses said.

Three people were killed and another injured at the Mukilteo property. State troopers pulled over and arrested the fleeing suspect on an interstate three counties away, authorities said.

Allen Ivanov, a 19-year-old incoming sophomore at the University of Washington, is in custody, Mukilteo officer Myron Travis confirmed to Daily Mail Online.

Travis said police are not looking for any other suspects.


Ivanov left cryptic tweets on his Twitter account just two days before the shooting.

'First and last tweet,' he wrote on July 28. 'I've been through it all.'

He then tweeted 'What's Ruger gonna think?', which seems to be a reference to Ruger Firearms.

Ivanov, who graduated from Kamiak High School in 2015, also posted a photo on Instagram that showed a rifle on the ground, with three bullets on the side.

Mukilteo resident Susan Gemmer said the shooter and one of the victims had recently broken up.

Gemmer said her 18-year-old granddaughter Alexis had been at the party with friends who attend Kamiak High when the shooter arrived with a rifle.

"She was hiding in the closet and called me from the closet while it was going on," Susan Gemmer said of her 18-year-old granddaughter, Alexis.

"We were texting back and forth, telling her to stay quiet, stay calm, we're on our way. She kept saying, 'They're dead, they're dead, I saw them, I was right there and I saw them.'"

Gemmer said that according to her granddaughter, the gunman arrived with a rifle at the party of about 15 to 20 friends from Kamiak High School - mostly recent graduates aged 18 to 20.

He walked through the house to the fire pit out back, where he shot two of the victims. He then made his way onto the roof, where some of the friends were hanging out, Gemmer said.

The young man who lived at the home tried to lead Alexis Gemmer to safety by escaping out the garage. As they rolled under the garage door and the boy bolted across the street, the gunman began shooting at him from the roof, her granddaughter told Gemmer.

"She panicked and ran back in the house and hid in the closet until police arrived," Gemmer said. The young man made it across the street.

"Our community has suffered a great loss tonight," Mukilteo Mayor Jennifer Gregerson said. "There were many young people who saw and heard things that no one should ever experience."

The shooting happened in the upscale Chennault neighborhood of Mukilteo, a waterfront town of about 20,000 people, 40km north of Seattle.

Washington State Patrol Trooper Will Finn said the male suspect was pulled over at around 2am heading south on Interstate 5 near Chehalis, about 180km away.

Finn declined to release information about his identity, but said troopers returned him and the vehicle he was driving to the custody of Mukilteo police.

The fatal shooting follows numerous ongoing gun-related incidents in the United States, including a shooting in a Florida teen club on Monday in which two people were killed and 17 injured.

Mukileo mayor Jennifer Gregerson said initial reports about the injured person are "hopeful".