UPDATED 3:10 PM: CAMBRIDGE, Mass.  — Students cleared to return to all 4 Harvard campus buildings evacuated over explosives report.

UPDATED 2:15 PM: CAMBRIDGE, Mass. — Harvard University has reopened a third building that was evacuated because of a report of possible explosives on campus.

Students have been told that they may return to the classroom building. Harvard Yard has also re-opened.

Two other buildings, a dorm and another classroom building, reopened earlier. A fourth building, the school’s Science Center, remains closed.

A search has yet to turn up any explosives.

An alert went out shortly after 9 a.m. Monday telling students at the Ivy League school outside Boston to leave the four buildings.

It’s exam time at Harvard and some students were just about to start finals when the alert went out.

The FBI is investigating along with Cambridge and university police and other agencies.

UPDATED 1:26 PM: CAMBRIDGE, Mass. — Harvard University has issued an all-clear for two of the four campus buildings evacuated because of a report of possible explosives.

Students have been told that they can return to Thayer Hall, which is a dorm, and Emerson Hall, a classroom building.

The two other buildings, the Science Center and another classroom building, remain closed. A search has yet to turn up any explosives.

An alert went out just after 9 a.m. Monday telling students at the Ivy League school outside Boston to leave the four buildings.

It’s exam time at Harvard and some students were just about to start finals when the alert went out.

The FBI is investigating along with Cambridge and university police and other agencies.

EARLIER UPDATE: CAMBRIDGE, Mass. — Four buildings on Harvard University’s campus near Boston were evacuated Monday after campus police received an unconfirmed report that explosives may have been placed inside, interrupting final exams.

Three of the evacuated buildings — one a dorm, and two classroom buildings — border Harvard Yard, and the other is the science center at the Ivy League school in Cambridge. Many of the students who had to leave went to another campus building, according to the school’s paper, The Harvard Crimson.

Both school and city police were investigating, along with officers from other agencies.

“Out of an abundance of caution, the buildings have been evacuated while the report is investigated,” the school said in a statement. “Harvard’s focus is on the safety of our students, faculty and staff.”

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.”

Sophomore Santiago Pardo said by phone that he and his roommate were keeping close tabs on the situation from their dorm, Adams House, which is not near Harvard Yard.

“We feel safe,” he said. “We’re not scared.”

The school tweeted that it would let students know as soon as possible whether afternoon exams would be given as scheduled.

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.