PITTSBURGH — Campus police, federal authorities and experts are stumped by more than 20 bomb threats since mid-February that have prompted building evacuations on the University of Pittsburgh campus, caused some professors to move classes or offer them online and led some students to stay off-campus.

Although the threats received more attention after a gunman fatally shot one person and wounded several others before he was shot dead by campus police at a University of Pittsburgh Medical Center psychiatric hospital on March 8, the string of threats actually began on Feb. 13.

At first, the threats were scrawled on bathroom stalls. Lately, they’ve been emailed — sometimes to reporters for the city’s two major newspapers, the Pittsburgh Post-Gazette and Pittsburgh Tribune-Review.

Two new threats against sets of buildings were received Thursday, one at 10 a.m., the other about 12 hours later. Police and experts say the sender is using an online “remailer” that relays e-mails through several intermediaries so the origin of the message cannot be traced, though some e-mails apparently were routed through computers in Austria.

“It would be very difficult if not impossible to trace this,” said Lorrie Cranor, who oversees Carnegie Mellon University’s CyLab Usable Privacy and Security Lab a few blocks from the Pitt campus.

No bombs have been found and nobody has been injured, though some are beginning to question the futility of building evacuations and searches.

“All this is still worth it,” campus police Chief Tim Delaney said earlier this week. “I will never put kids in danger.”

Andrew Sours, 21, a junior from the nearby suburb of Jefferson Hills, said one of his professors is holding class online as a result.

“Because it happened so many times, it spooked people, and they emailed the professor and said they’re scared to come to class,” Sours said.