PITTSBURGH (AP) — Dozens of bomb threats at the University of Pittsburgh, including at least four on 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.

"But I think just shutting down the university would be a mistake," he said, adding that's probably what the person making the threats wants.

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 at about 4 a.m.

Student Dawn Diehl, who's studying for a master's degree in library science, said it wasn't until a few days ago that the bomb threats started to affect her in terms of "my feelings of security."

"So now it's pretty alarming," she said. "We've never had an experience like this. I kind of have that feeling like, where's this going to end?"

Diehl was surprised Monday to find all but one door to the main library locked and everyone's bags being searched.

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 U.S. 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 with making them.

Fifth-year chemistry student Brian Graham said the first threats were written on walls in buildings and he hoped security officers would catch the perpetrator. But, he said, threats then started arriving by email.

"I think it's a little bit more nerve-racking," Graham said of the latest wave of threats. "I have to either stay later or come in different hours. I would be about to leave home, and then there'd be a bomb threat."

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Graham says he's confident that Pitt officials are doing what needs to be done to protect students and find the person responsible.

"It seems that they're taking all the appropriate steps," he said.

The threats have caused some professors to move classes outdoors or offer them online and have led some students to stay off-campus. The university, located a few miles from downtown Pittsburgh, has about 3,800 full-time faculty members who serve 34,000 students.

The university is urging faculty members to make arrangements for students to make up classes or exams missed because of evacuations, but it says there are no plans to end the semester early.

No bombs have been found, and nobody has been injured, but police say the building evacuations will continue if warranted. There have been about 25 threats targeting numerous buildings, with some of those threatened multiple times.

U.S. Attorney David Hickton in Pittsburgh issued a statement Friday commending Pitt's response and confirming the threats "are being vigorously, aggressively and thoroughly investigated through every possible mean" by the region's Joint Terrorism Task Force, which includes the campus police and the FBI.