A “credible threat” closed Denver’s East High School on Friday afternoon. Manual High School also was closed early, according to 9News.

School officials released students at 12:15 p.m., and all school activities were cancelled. No one was reported injured. A robocall to parents stated that “all students are safe at this time.”

The Denver police were working with the school to investigate the threat, a police spokesman confirmed. Police officers assisted in the dismissal, according to the message to parents. Patrols remained in the area after students were dismissed.

The police wouldn’t specify how the threat was made or its contents. However, FOX31 reported that parents believe the threat was gang-related.