PITTSBURGH -- The principal of MonessenHigh School said students were released early on Thursday after someone called in a threat to the school.

School officials said the call came in around 11:30 a.m. The principal said a man on the line stated that the school was going to be "vaporized" at 3 p.m.

The principal decided to begin dismissals at 1 p.m. and by 1:35 p.m. all students and staff were out of the building.

School police, police, firefighters and the mayor met at the school to search the building, officials said. As a precaution, school officials cancelled after-school activities.

School officials said the phone call could have been a senior prank and authorities are working on tracing the call.