PITTSBURGH — Dozens of bomb threats at the University of Pittsburgh, including at least four Monday, have made professors start holding classes outside and forced security officials to put in new building access measures and offer a $50,000 reward for information.

Some students “are definitely afraid,” said Brian Haughwout, a junior who had one of his final exams changed to a take-home because of the disruptions.

The threats began in mid-February, at first targeting a landmark building at the center of campus. But in recent weeks numerous buildings have been threatened. Four threats had been made by mid-afternoon Monday, starting about 4 a.m.

Under new security measures, students and faculty members will need school IDs to get into buildings. Non-residents won’t be permitted in dormitories.

University police, the FBI and the Secret Service have said they have a person of interest in the investigation. Authorities say some of the threats have been traced to or through computers in Austria, but nobody has been charged.