The Honolulu Police Department is investigating a first-degree terroristic threatening case after a written bomb threat scrawled on a poster prompted an evacuation of Mililani High School on Thursday.

The threat was found at 10:20 a.m., and all buildings were evacuated until 11 a.m., when the all-clear was issued and students returned to their classes, according to Principal Fred Murphy. Police said in a statement that “no suspicious persons or devices were found.”

“The Honolulu Police Department responded immediately,” Murphy wrote in a letter to parents today. “They investigated, a full sweep was conducted and it was determined that there was no credible threat.”

The threatening note, written in red at the top of a poster advertising volleyball tryouts, said, “The school will be bombed at 1:00.”

There was some confusion in the community when the Department of Education put out a tweet that described the event as an “evacuation drill” and noted that “HPD is assisting.” It later clarified the situation by posting the principal’s letter. Asked about the wording of the tweet, Donalyn Dela Cruz, DOE communications director, issued the following statement to the Honolulu Star-Advertiser:

“This morning Mililani High School received a threat,” she wrote. “School administrators called for an evacuation and notified the Department that an evacuation drill was taking place. The Department tweeted this information and provided updates to the situation as it became available.”

Murphy urged parents to discuss the seriousness of such threats with their children and to share any information that could help with the investigation. The Police Department is asking anyone with information to contact CrimeStoppers at 955-8300 or HPD.