An explosion at an empty classroom at Yale University's law school caused some damage but no injuries, officials at the Ivy League school said.
The FBI said an "explosive device" had gone off. The blast, which knocked out a wall between two rooms, came a day after the US government raised its terror alert status to "high" from "elevated" because of what officials said was a renewed risk of terrorist attack in the United States.
President George W. Bush, who graduated from Yale, spoke earlier yesterday at the Coast Guard Academy in New London, Connecticut, about 50 miles east of New Haven, where Yale is based.
The president's daughter Barbara attends Yale University as an undergraduate student but was not on campus at the time of the blast.
There had been no threat before the explosion and no claim of responsibility afterward.
The explosion came at the end of the school's academic year. Dormitories on campus were empty, most students had finished their exams and the law school's graduation ceremonies had been set for Monday.