The Friday (local time) incident at Marysville-Pilchuck High School north of Seattle sent students fleeing from the building and sheltering under desks in the latest of a series of school shootings.
County Sheriff's Office said on Saturday that officers had completed their on-scene investigation and recovered a 0.40 caliber handgun.
Police have not released the shooter's identity, but a Marysville school official and multiple classmates said a well-liked freshman named Jaylen Fryberg was responsible for Friday's rampage.
Students who knew Fryberg described him as an outgoing and popular football player, unlike the loner personality that is often associated with school shootings.
Witnesses said the shooter had targeted a single table in the cafeteria.
"He came up from behind and had a gun in his hand, and he fired about eight bullets," student Jordan Luton told CNN. " ... They were his friends so it wasn't just random."
All five victims were under 18, medical officials said.
One female classmate was killed, police said. Her identity has not been released.
Fifteen-year-old Andrew Fryberg was shot in the head and remained in critical condition in intensive care at Harborview Medical Center in Seattle on Saturday, a spokeswoman said. She said Nate Hatch, 14, was shot in the jaw and was in serious condition there.
Local media, community members and fellow students said the boys were cousins of the shooter and members of the Native American Tulalip Tribes.
The two female victims, Shaylee Chuckulnaskit, 14, and Gia Soirano, 14, remained in critical condition on Saturday at Providence Regional Medical Center in Everett, said Dr. Joanne Roberts.
"The next three days are going to be critical," she said.
The sheriff's office said a cafeteria worker had attempted to stop the shooter, who took his own life.
SEARCH FOR A MOTIVE
Authorities were still piecing together a motive, but a school official and several witnesses said Jaylen Fryberg had been in a fight with another student. A friend of one of the female victims said he had been recently rejected by a girl who favored his cousin.
"I heard he asked her out and she rebuffed him and was with his cousin," said Bella Panjeli, a ninth-grade student at a different school who said she was a friend of one female victim.
She added that she learned of the connection after talking to the victim's family and friends on social media. "It was a fight over a girl."
There were no indications on his social media accounts that he had been planning such a rampage, but on Tuesday he posted his feelings of despondency, apparently over a romantic split, on Twitter.
"It breaks me...It actually does...I know it seems like I'm sweating it off... But I'm not.. And I never will be able to," he wrote.
Students who were evacuated to safety on Friday were slowly returning to the school building Saturday to pick up items they left behind.
Grief counselors were going to be available to both students and educators, the district superintendent said in a statement.
Classmates said they were shocked that mass violence had come to their city.
Karalyn Demarest, 17, a senior at Marysville, added a bouquet of flowers to nearly a dozen that were lying against a fence at the school.
"I was in class in a classroom next to the cafeteria," she said. " ... I just heard screaming.
"It's just really sad and tragic. You just never expect it to happen anywhere near you."
