A bomb scare forced Harvard University Monday to evacuate four buildings, call in police and cancel final exams underway at the elite US university in the northeastern town of Cambridge.
The university ordered the evacuations at 9:02 am at the Science Center, the Thayer dormitory, the Sever classroom and lecture hall and the Emerson building, home to the philosophy department.
Harvard tweeted that there had been "NO reports of explosions" but police were immediately scrambled to the sprawling, snowy campus where witnesses tweeted of helicopters circling overhead.
But four hours after the original alert, the university said two of the buildings had been cleared and students could return.
Sever and the Science Center, which is home to the mathematics, statistics and history of science departments, were still closed, it announced on its website.
Federal and state officials have also been drafted in to assist with the investigation although none of the reports of explosives have yet been confirmed.
Harvard University police have "no reason to believe there is a threat to any other site on campus," the university said.
The Ivy League school is one of the most prestigious universities in the world. It has around 21,000 students and dominates the Massachusetts town of Cambridge near Boston.
Sam Weinstock, incoming president of student newspaper the Harvard Crimson, told CNN that final exams scheduled on Monday in three of the affected buildings had been canceled.
"Students were removed from those exams, taken to the freshman dining hall where they were told that exams would be canceled," he said.
Asked about growing media speculation on whether the call could have been a hoax, Weinstock refused to speculate.
"We have no idea what the nature of the call was or what the nature of the threat is now," he told CNN.
University of Massachusetts Boston also announced a brief evacuation of one building Monday over reports of a possible gunman. The alert was declared false within minutes.
Monday's scares came three days after an American schoolboy armed with a shotgun opened fire and wounded two fellow students before killing himself at a high school in Colorado.
They also came less than a month after Harvard's great rival Yale went on a four-hour lockdown after a hoax caller claimed his roommate was planning to shoot people on the campus.
Teams of police scoured the sprawling Yale campus in New Haven, Connecticut but there were never any reports of shots fired nor of any injuries. Nor was the presumed gunman found.
Harvard was founded in 1636 and has educated current and former leaders from all over the world, including a long list of American presidents such as Barack Obama and John F. Kennedy.