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CAMBRIDGE, Mass. (AP) -- Four buildings on Harvard University's campus near Boston were evacuated for hours Monday, interrupting final exams, after police received a report that explosives may have been placed inside. The buildings were reopened after hours of searches.

The FBI was investigating the report along with Cambridge and university police and other agencies, and the school said the buildings were evacuated out of an abundance of caution.

"Harvard's focus is on the safety of our students, faculty and staff," the university said in a statement. It would not immediately say after reopening all the buildings whether any explosives were found.

Three of the evacuated buildings - one a dorm, and two classroom buildings - border Harvard Yard, and the other was the science center at the Ivy League school in Cambridge.

The mood on campus was calm as students streamed out of Harvard Yard on a frigid morning with temperatures in the 20s. The gates around the yard were closed and people were allowed to leave but not enter unless they had school IDs.

Juniors Alexander Ryjik of Alexandria, Va., and Diego Abrahao of Sunny Isles Beach, Fla., said their professor was handing out exam booklets for their Politics of American Education class shortly after 9 a.m. when the alert went out that they had to leave Emerson Hall.

"I have a good guess somebody called it in so they wouldn't have to take an exam," Ryjik said. "It's frustrating because now the exam will have to be postponed."

A classroom building was also briefly evacuated Monday at the University of Massachusetts-Boston, which has 16,000 undergraduates and graduate students who are also taking final exams this week. University police got a call from someone who said they had seen a person with a gun in the building, which was closed while university, Boston and state police searched it. They determined there was no one with a gun and the call is being investigated, said school spokesman DeWayne Lehman.

Last month, another Ivy League school, Yale University in Connecticut, was locked down for nearly six hours while authorities investigated a phone call saying an armed man was heading to shoot it up, a warning they later said was likely a hoax.

And in February, someone called in a hoax about a gunman on the campus of the Massachusetts Institute of Technology, another elite school about two miles from Harvard. The university said the gunman was a staff member looking for revenge after the suicide of an Internet activist accused of illegally using MIT computers.

Associated Press writer Matt Small in Washington contributed to this report.

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