(CNN) -- The home of the University of Pittsburgh's chancellor was among the latest targets of a series of empty bomb threats plaguing the Pennsylvania school, officials said Wednesday.
The Tuesday night scare at the home of Mark Nordenberg was followed by five other threats on university buildings early Wednesday morning, according to school spokesman Robert Hill.
A total of 55 buildings on the Pittsburgh campus have received dozens of threats since February 13, he said. Some buildings have faced more than one threat.
All of the threats have since been cleared, and authorities have yet to uncover explosives in any of the incidents.
The Department of Justice, the FBI and the University of Pittsburgh police are investigating. A federal law enforcement official described the investigation as "extremely focused" but would not say whether investigators have a specific suspect or suspects.
Many of the earlier threats were written in bathroom stalls, while most of the more recent ones have been made against Pittsburgh's newspapers -- the Post-Gazette and the Tribune-Review .
The phenomenon has also affected campus life.
"I'm running on two hours of sleep," said Pitt senior Dmitry Kalika, who was evacuated from his dorm early Wednesday.
He said that just after a bomb scare, he stayed with fellow student Nick Czarnek, who is part of an emerging network that uses social media to offer temporary housing for displaced students. More than 150 apartments and dorms are listed in the forum, which was created by students and posted on Facebook.
"I think it's been pretty effective," Czarnek said.
Students and parents, meanwhile, have expressed weariness and worry about the threats.
"The first few were kind of spread out, and they were seen as a joke," Czarnek said. "But after they started increasing in frequency, now everyone is sort of creeped out."
"The phone is ringing all the time," said Czarnek's mother, Victoria, who signed up for the university's mobile alert system.
"I have been very much on edge," said Rochester resident Donna Curran, whose son is a student at the university. "It's unnerving. ... He texts me every time there is a threat."
"It's gotten to the point where it's scary," said her son, Chris. "At first, people were just annoyed."
He said that one of his professors offered to end class for the rest of the semester and calculate grades based on current performance. Another professor, he said, is conducting the class's final online as a result of the threats.
In a letter posted on the University of Pittsburgh website Friday, Chancellor Nordenberg said a seemingly "unending succession of bomb threats" have been "a disruption to life on campus." He said that despite the inconvenience, university officials have evacuated every threatened building each time.
The university heightened security around campus Monday by limiting access to buildings and establishing security checkpoints.
Students are being encouraged to carry "only those items necessary for your school or work" and will have to show an identification card to enter campus facilities.
In addition, no parents or other nonuniversity residents will be allowed in residence halls.
The university is also offering a $50,000 reward for information that leads to the arrest and conviction of those responsible.
In his online message, Nordenberg addressed concerns from people who feel the university is being too cautious, and others who believe the campus should be shut down.
"Our highest priority has been the safety of the Pitt community," he said.
The Justice Department recently issued a statement about university policies in response to the threats.
David Hickton, U.S. attorney for the Western District of Pennsylvania, said the school is "exercising appropriate regard for safety" while refusing to allow the bomb threats to "paralyze the entire University community in its pursuit of learning and teaching."
The university is providing counseling to help students and community members deal with the frustrations and uncertainties caused by the threats.
CNN's Antoinette Campbell, Carol Cratty and Pauline Kim contributed to this report.