Story highlights Justice Department, FBI, school police investigating threats that begin on February 13

The university began limiting campus building access Monday

No evidence of explosives has been found on campus

The University of Pittsburgh heightened security Monday while school and federal officials continued to investigate bomb threats made against the campus.

The series of threats have led to the constant evacuations of buildings on campus and "an understandable source of heightened anxiety," University Chancellor Mark Nordenberg said Friday in a statement on the school's website.

On Monday, the university said it would begin limiting access to buildings that have already been swept and given an all-clear. The new restrictions will be extended to all campus buildings in the next few days, according to the statement.

The Department of Justice, the FBI and the University of Pittsburgh police are looking into the incidents.

The bomb threats "are being vigorously, aggressively and thoroughly investigated through every possible means," said David Hickton, U.S. attorney for the Western District of Pennsylvania.

On Saturday, students were told that the university would remain open despite the continued threats and that alternate accommodations were being made to allow them to complete their coursework.

Administration officials have placed two security checkpoints at single entrances to buildings. One is for students carrying backpacks, book bags and packages, and the other is for those without any items.

Students are being encouraged to carry "only those items necessary for your school or work" and will have to show an ID card to enter campus facilities.

In addition, no parents or nonuniversity residents will be allowed in residence halls.

The university has asked that witnesses share any information about the bomb threats. It is also offering a $50,000 reward if that information leads to the arrest and conviction of those responsible.

Although no evidence of explosives has been found on campus, the university has said it will continue to evacuate and search every targeted building.

In his website 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 supports the way the administration is handling the situation.

According to Hickton, the university 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.