BOSTON - Harvard University on Monday evacuated four buildings at the heart of its centuries-old campus in Cambridge, Massachusetts, after warning students of unconfirmed reports that explosives had been planted.The Ivy League school said it evacuated the Science Center, Thayer, Sever and Emerson Halls."Out of an abundance of caution, the buildings have been evacuated while the report is investigated," Harvard said on its Web site.Harvard and Cambridge Police officials did not immediately return calls seeking further details.Harvard's campus is located in a crowded urban area adjoining Boston. Students this week were scheduled to take final exams.The school, founded in 1636, has about 21,000 students.It was the latest in a series of security scares at US schools and universities. Three days ago a Colorado high school student intent on confronting a teacher opened fire at his school, severely wounding a classmate before killing himself.Late last month Yale University in New Haven, Connecticut, placed its campus on lockdown for most of a day, after an anonymous caller warned officials that his roommate was headed to the school planning to shoot people. No gunman was found.