Dozens of bomb threats at the University of Pittsburgh are forcing security officials to enact new building security measures and offer a $50,000 reward for information, according to a report by The Associated Press

These disturbing disruptions are becoming more frequent, with at least four on Monday. Professors have started holding classes outside or switched to online classes and take-home exams. On Monday, all but one door to the main library was locked, and every bag was searched.

Under the new security measures, students and faculty will require school IDs to get into buildings, and non-residents will not be permitted in dormitories, says AP.

The threats began in mid-February, first targeting a landmark building at the center of campus, but recently numerous buildings have been threatened. Threats were first written on walls inside of buildings, but recently they are arriving by email. So far, no bombs have been found and no one has been injured. The university is urging faculty to make arrangements for students to make up classes because of evacuations, but it says that there are no plans to end the semester early. 

The university is a few miles from downtown Pittsburgh, and has about 3,800 full-time faculty members and 34,000 students. 

University police, the FBI and U.S. Secret Service have said that they have a person of interest in the investigation, and authorities say some threats have been traced to or through computers in Austria, AP reports. No one has been charged.

There have been 25 threats in total.