NATCHITOCHES, La. (AP) — Several buildings were evacuated for a time at Northwestern State University and classes were canceled Monday after a college-prepatory high school housed on the campus received a bomb threat.

Officials said later Monday that no explosives were found and both the university and high school were declared safe.

“Thank God there was no incident,” said Pat Widhalm, principal at the Louisiana School for Math, Science and the Arts. “No explosives were found.”

The evacuation and lockdown lasted for just over two hours and classes were to resume as usual on Tuesday, campus spokesman David West said. Some 4,500 students were affected by the evacuation order and Widhalm said his school has 301 students from throughout the state who also live on campus.

Widhalm said it all began about 10:45 a.m. when a student at the school of 10th through 12th graders, brought a note to an administrator that had been taped on a mirror.

“The typed note said something to the effect of there were explosives in the main academic building that will go off at 1 p.m. and this is not a joke,” he said. “We immediately called campus police, which were here within five minutes. They called Louisiana State Police.”

Widhalm said authorities began a “door-to-door, room-to-room, office-to-office” search of the high school’s main academic building — the target of the threat. The school also occupies three other buildings on the campus.

Students were sent to their dorms and staff and faculty were sent home, he said.

University police also evacuated nearby campus buildings, including Fournet, Morrison, Kyser, Russell, Williamson and Bienvenu halls; the Family and Consumer Sciences Building, Watson Library; the A.A. Fredericks Center for Creative and Performing Arts, the NSU Alumni Center and the Friedman Student Union.

At least one entrance to the campus was closed at the height of the threat.

The threat is being investigated.

“We take every possible threat seriously,” Widhalm said. “The students were safely ensconced in their dorms. Everyone was safe and secure. It’s such a violation to a community when threatened like this. You better believe we will cooperate fully with campus police as they investigate.”

Students apply to the college-prep school during either their freshman or sophomore year for the opportunity to attend their final years of high school. There is no tuition but students pay a fee for room and board that sometimes be reduced or waived.

Students earn a up to a year-and-a-half worth of college credits, Widhalm said.