Jordan Luton was finishing his lunch in the cafeteria at Washington state's Marysville-Pilchuck High School on Friday when he heard it -- a loud bang.

Then there was another. And another. And another. And another.

What he saw was freshman Jaylen Fryberg go up to a table with students, "came up from behind ... and fired about six bullets into the backs of them," Luton told CNN. "They were his friends, so it wasn't just random."

Some of the students began to run. Luton and others hit the ground, some hiding under tables.

By the time it was over, two people -- the gunman and a female student -- were dead and four were wounded, according to authorities. Those wounded were all under the age of 18, they said.

The shooter died of a self-inflicted gunshot wound, Marysville police spokesman Robb Lamoureux told reporters.

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Two girls are in the intensive care unit at Providence Regional Medical Center in Everett, and two boys are in ICU at Harborview Medical Center in Seattle, Providence spokeswoman Erin Al-Wazan said.

Three are "very critically ill" with "very serious" injuries, she said. One is in serious condition. One of the boys, age 14, suffered a jaw injury. The other, age 15, was critically injured in the head.

Source: Gun traced to shooter's father

The gun used in the shooting has been traced to Fryberg's father, a law enforcement source with knowledge of the investigation told CNN. It is a "high capacity" weapon but did not have an extended magazine, the source said on condition of anonymity.

The source said investigators are searching the family home.

A Beretta .40-caliber handgun is believed to have been used, a federal law enforcement source told CNN.

Why did this happen? That's the question being asked as investigators combed the school late Friday for clues and interviewed students who witnessed the shooting.

Investigators were expected to work through the night at the school, which has been closed through next week.

While authorities have not detailed what occurred inside or identified the shooter, students offered up accounts that painted a terrifying, chaotic picture.

The shooting began during an early lunch break at the school of about 2,500 students that sits in the heart of Marysville, a small city bisected by the major thoroughfare connecting the region with Vancouver, British Columbia.

Authorities got a 911 phone call at 10:39 a.m. (1:39 p.m. ET) from someone inside the school, which is north of Seattle and Everett, Lamoureux said.

'People started screaming'

Luton was sitting two tables away when the shooting began.

Fryberg appeared to target one table, where there were about 10 students sitting. "I'm pretty sure he shot at specific people because they were pretty good friends with him," Luton said.

"He put some bullets in the back of the students," he said. "He turned and looked at me and my girlfriend ... and kind of gave us a smirk."

The shooter continued firing as he left the cafeteria, Luton said.

"I have no idea what his motive was because yesterday at football practice, he was all fine, talking .... having a good time," he said. "And then today, just horrible. I don't know what went through his head or what caused him to do it."

By all accounts, Fryberg was a popular student. Just a week ago, he had been named as the high school's freshman homecoming prince, according to a YouTube video of the ceremony and accounts provided by students to CNN.

Fryberg's multiple social media accounts depict him frequently hunting and using rifles. Those accounts say he was a Native American and a member of the Tulalip tribe.

Luton could not confirm reports that Fryberg had been bullied. But two weeks ago, according to Luton, Fryberg got into a fight after somebody said "something racist" to him.

'Run, get out of here'

Freshman Rachel Heichel was sitting at a table about 60 feet away from Fryberg.

When the shots started, she turned around and saw the shooter standing there.

"When I saw him, I was like 'Oh my God, that's Jaylen.' I would have never expected it would have been him, out of all people," she said.

"It was really heartbreaking for me to see that. I saw him with the a gun in his hand, shooting."

She never heard Fryberg say anything.

"All I heard was people screaming, 'Run, get out of here. Let's go,'" Heichel said.

At first, they thought it was a fire drill

A student locked down inside a classroom told CNN by phone that students initially thought it was a fire drill until teachers told everyone to run into classrooms because there was a shooting.

A student who says he was in the cafeteria when the shooting began told CNN he called the police, and then gave his phone to a teacher.

The student described a chaotic and bloody scene, saying blood was "everywhere."

Another student, identified as Austin Taylor, told CNN Seattle affiliate KING that he had just finished eating when he saw the shooter.

"He was just sitting there. Everyone was talking. All of the sudden, I see him stand up, pull something out of his pocket," he said.

"At first, I thought it was just someone making a really loud noise ...like a big loud pop ... until I heard four more after that. And I saw three kids just fall from the table."

Austin said he ducked under a table. When the shooting stopped, he said he looked out and saw the shooter was trying to reload.

"When that happened, I just ran in the opposite direction, and I was out of there as fast as I could," he said.