PENDLETON (AP) — The discovery of a backpack following a bomb threat emptied classrooms at Pendleton High School.

Pendleton Police Lt. Mark Swanson says the threat was placed Monday morning from a pay phone on campus. Police officers and school administrators then conducted a sweep of the school and found a backpack in a trash can.

A robot and an Oregon State Police bomb squad technician examined the backpack and found it empty.

Following a second search, students were sent back to class.

Police are trying to find the person who made the false threat.