CAMBRIDGE (CBS) – Harvard University evacuated four buildings on campus Monday morning after receiving unconfirmed reports of explosives.

All four were cleared after several hours of searches and re-opened in the afternoon.

No suspicious devices were found.

It all began just after 9 a.m. when Harvard tweeted this urgent message:

Alert: Unconfirmed reports of explosives at four sites on campus: Science Center, Thayer, Sever, and Emerson. Evacuate those buildings now. — Harvard University (@Harvard) December 16, 2013

The university followed with another tweet saying, “There have been NO reports of explosions.”

Cambridge police told WBZ-TV several people affiliated with the university received emails about the bomb threats and that prompted the alert.

State, local and campus police conducted a search of the three academic buildings and one residence hall, but apparently nothing was found.

Access to Harvard Yard was also restricted to residents for several hours.

“As of the writing of this message the report remains unconfirmed and the HUPD has no reason to believe there is a threat to any other site on campus,” the school said in a statement at 10:40 a.m.

Students are still on campus for exams this week. Some told WBZ-TV they were just being handed their tests when an alarm went off and the evacuation began.

At first they thought it was just a fire alarm and most people left their bags behind.

“Police (said) go find someplace warm because you’re not going to be able to come back anytime soon,” an unidentified student said.

Several students were moved into a freshman dining hall.

According to the Harvard Crimson, in email to colleagues, Administration Board Secretary Jay Ellison said pass/fail grades will count for general education and concentration requirements.

Several professors rescheduled exams for Monday or Tuesday evening. Students were also told they can opt to take the exams during the third week of the spring semester or let their work-to-date be used to determine their final grade.

Harvard says the exams scheduled for 2 p.m. Monday, were to be held as scheduled, “except for: CB 49; Econ 2010c; and Societies and the World 50.”

There have been no reports of any injuries.

In an email to the Harvard community Monday afternoon, Executive Vice President Katie Lapp said, “Safeguarding our community in this instance unfortunately required the disruption of exams and the evacuation of one of our freshman dormitories. The HUPD, in close cooperation with local, state and federal agencies, is continuing to investigate this incident to determine who may be responsible.

A White House official told CBS News that President Obama has been briefed about the situation at Harvard, where he attended law school.

“We continue to encourage the public to follow directions from the local officials. The White House will remain in touch with our federal, state and local partners and the President will continue to receive briefings as needed,” the official said.

University employees were also alerted via automated phone message this morning.

No other details are available at this point in the investigation.

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