Third victim of Washington school shooter dies

A 14-year-old US girl injured during the recent Washington state school shooting has died in hospital.

"Shaylee Chuckulnaskit, known affectionately as Shay to her family and friends, passed away at Providence Regional Medical Center as a result of severe injuries from last Friday's school shooting," the hospital said in reference to the assault in the northwestern state of Washington.

The gunman, identified as Jaylen Fryberg, fatally shot one girl on the spot before turning the gun on himself. Another girl succumbed to her injuries in hospital last Sunday.

In a brief statement, members of Chuckulnaskit's family said their hearts were "broken at the passing of our beautiful daughter."

"She has been a loving daughter, a caring sister, a devoted friend and a wonderful part of our community. We can't imagine life without her," they said.

Fryberg is believed to have targeted a particular group of students during his shooting, including friends and his cousin.

Another boy is in a serious condition following the rampage, having been shot in the jaw.

Police are attempting to ascertain a motive for for the incident at Marysville-Pilchuck High School in Marysville, north of Seattle.

The teen widely identified as the shooter, Jaylen Fryberg, had left a series of tortured posts on Twitter, suggesting he was used to handling guns, and hinting that a failed romance may have led to the shooting.

Local media said Fryberg, a Native American, played on the school football team and had been named a homecoming prince just a week before.