MOUNTLAKE TERRACE — A Snohomish County high school senior accused in an attack that broke the jaw of a special education student has been arrested and expelled.

Edmonds School District officials say the 18-year-old Mountlake Terrace High School student was arrested Tuesday for investigation of assault and robbery in the attack in a school restroom.

The injured 16-year-old student reportedly also suffered cuts and bruises and lost a tooth and his wallet.

The Daily Herald reports that relatives told school officials Wednesday that the injured student has been released from Harborview Medical Center in Seattle.

High school Principal Greg Schwab told parents in a letter that the students did not know each other. No weapon was involved.

Authorities said there’s no clear indication of a motive.