Jaylen Fryberg, who witnesses said gunned down students at Marysville-Pilchuck High School, is cousins with two of the wounded, their grandfather said.
"All three of them are cousins, and they live right close to each other," Don Hatch told CNN affiliate KOMO.
By the time the shooting was over, two people -- the gunman and a female student -- were dead and four were wounded, according to authorities.
The victims were all under age 18, they said.
The shooter died of a self-inflicted gunshot wound, Marysville police spokesman Robb Lamoureux said.
Two girls are in ICU at a hospital in Seattle while two boys are in intensive care unit at a hospital in Everett, said Erin Al-Wazan, a spokeswoman for the latter facility.
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.
Lunchroom in shock
Authorities in Maryville, which is north of Seattle, have not provided details on the shooting or identified the shooter, but students offered up accounts that painted a terrifying, chaotic picture.
Fellow student Jordan Luton was finishing his lunch in the cafeteria at the school in Marysville when he heard a loud bang.
Then there was another. And another. And another. And another.
What he saw was freshman 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."
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.
Student Austin Taylor told CNN 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."
School in scramble
Luton was sitting two tables away when the shooting began. "He (Fryberg) turned and looked at me and my girlfriend," he said, "and kind of gave us a smirk."
Luton and others hit the ground, some hiding under tables.
Austin said he ducked under a table, too. Then the gunfire paused, and 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.
Others ran out, too.
The shooter left the cafeteria and continued firing, Luton said.
In the hallways, the rest of the school went into a scramble.
"All I heard was people screaming, 'Run, get out of here. Let's go,'" Heichel said.
Teachers herded students into classrooms and ordered a lockdown.
A student locked down inside a classroom told CNN by phone that students initially thought it was a fire drill.
Many of them ran out of the building.
Police secure school
Someone called 911, and by 10:40 a.m. local time, police swarmed the building, guns readied.
They went room to room, placing tape over doors to mark the ones they'd secured.
"I've got 30 students at the gym ready to go to the northwest," police could be heard saying over radio. "We're going there to evacuate more students.
Then they came upon the gunman's dead body.
One student told CNN affiliate KIRO that a teacher stopped the gunman from firing on more students when she briefly grabbed his arm, said student Erick Cervantes. The gunman pulled away. "Then I heard another shot," Cervantes said. He believes it was the shot that killed the gunman.
The gun used in the shooting has been traced to Fryberg's father, a law enforcement source told CNN.
The source, who spoke on condition of anonymity, 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.
A sheriff's spokesman said investigators at the scene would secure more evidence. It is closed through next week.
It was hard for Luton to make sense of the shooting.
"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 the high school's freshman homecoming prince, according to a YouTube video of the ceremony and accounts provided by students to CNN.
Multiple social media accounts in Fryberg's name depict him frequently hunting and using rifles. Those accounts say he was a Native American and a member of the Tulalip tribe.
But a Twitter account in his name gave a very different picture, that of a teenager with a broken heart.
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.
In the evening, tears streamed down cheeks as hundreds gathered at a nearby church for a vigil.
They wept, prayed and sang.