A bomb scare on Monday forced Harvard University to evacuate four buildings, call in police and cancel final exams underway at the elite US university in the northeastern town of Cambridge.
An alert on its website announced the evacuations at the Science Center, the Thayer dormitory, the Sever classroom and lecture hall and the Emerson building, home to the philosophy department.
"Please evacuate those buildings now," it said.
"Out of an abundance of caution, the buildings have been evacuated while the report is investigated. Harvard's focus is on the safety of our students, faculty and staff," it added.
Police immediately scrambled to the affected buildings.
But an hour and a half after the original alert, the report remained unconfirmed and police have "no reason to believe there is a threat to any other side on campus," the university said.
Harvard also tweeted that there had been "NO reports of explosions".
Harvard, one of the most prestigious universities in the world, dominates the Massachusetts town of Cambridge near Boston.
Harvard's Science Center is home to the mathematics, statistics and history of science departments. The university is currently in session, going into recess on December 21.
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.
Monday's scare comes 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.
It also comes less than a month after Harvard's great rival Yale went on a four-hour lockdown after an anonymous caller said 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 around 21,000 students. It has educated current and former leaders from all over the world and a string of American presidents, including Barack Obama.