Kentwood High School closed early on Friday after an email threat against students and staff.
The King County Sheriff's Office said a former student hacked into another former student's email account and sent the message, which threatened a "hostage take-over and a bloodbath."
A bomb squad and K-9 unit were called to the school to investigate, although Sgt. Cindi West said officials do not believe the threat was credible.
A spokesperson for the Kent School District told KIRO Radio there was enough "specificity" in the threat to prompt the district to dismiss students. All afternoon and evening activities were canceled.