Information from students and community members has helped them to focus on potential suspects, said U.S. Attorney David J. Hickton, who encouraged people who think they may have information to continue sharing it with law enforcement.
The Pitt threats began in mid-February, at first targeting a landmark building at the center of campus. But in recent weeks numerous buildings, including dorms, have been threatened.
On Tuesday night, a threat was made against Chancellor Mark Nordenberg's home.
No bombs have been found, and nobody has been injured, but police say building evacuations will continue if warranted. There have been about 25 threats, with some buildings threatened multiple times.
University police announced the arrest of a man at Pittsburgh International Airport on charges of harassment and terroristic threats but made no connection to the string of bomb threats. Pitt spokesman John Fedele told the Pittsburgh Post-Gazette that Mark Lee Krangle, 65, of Croton-on-Hudson, N.Y., was taken into custody at 1:30 p.m. Wednesday by campus police assisted by Allegheny County police. A listed number for him could not be found and it was unclear whether he had an attorney.
The school is offering a $50,000 reward for information and has imposed new security restrictions. Professors have started holding classes outside and offering them online, and some students have started staying off campus.
University police, the FBI and the U.S. Secret Service had previously said they had a person of interest in the investigation.
The first threats were scrawled on restroom walls, but more recently they have been sent by email to the school or to reporters for local newspapers. Authorities say some of the threats have been traced to or through computers in Austria, but nobody has been charged with making them.
Authorities say that people with tips have many options for reporting information and can remain anonymous.