A fifth teenager wounded in the Washington state high school shooting has died.

The latest fatality comes two weeks after the deadly rampage at Marysville-Pilchuck High School in which pupil Jaylen Fryberg, 15, opened fire with a handgun in the cafeteria on October 24, before dying of a self-inflicted wound.

Harbourview Medical Centre in Seattle said 15-year-old Andrew Fryberg, a cousin of the teenage gunman, died last night, local time.

The other victims were Zoe Galasso, 14, who was killed during the shooting; Gia Soriano, 14, who died on October 26 at Providence Regional Medical Centre in Everett; and 14-year-old Shaylee Chuckulnaskit, who died on October 31 at the Everett hospital.

Nate Hatch, 14, who was shot in the jaw, was released from hospital on Thursday.

The school, 30 miles north of Seattle, reopened on Monday after being closed for a week.

Snohomish County Sheriff Ty Trenary said last week Jaylen Fryberg invited his victims to lunch by text message, shot them at their table, then killed himself.

The sheriff said detectives were digging through reams of text messages, phone and social media records as part of an investigation that could take months.

"The question everybody wants is, 'Why?'," Sheriff Trenary said. "I don't know that the 'why' is something we can provide."

Fryberg, a football player who was named a prince on the school's homecoming court a week before the killings, was a member of a prominent Tulalip Tribes family.

He seemed happy although he was also upset about a girl, friends said. His Twitter feed was recently full of vague, anguished postings, like "It won't last ... It'll never last", and "I should have listened ... You were right ... The whole time you were right".

Nate was still in hospital when he posted a message of forgiveness on Twitter.

"I love you and I forgive you jaylen rest in peace," he wrote.