The blast late Wednesday caused no injuries and only minor damage, but jangled nerves in a nation on heightened alert for possible terrorist attacks.
The FBI's Joint Terrorism Task Force immediately responded to the scene along with agents from the U.S. Bureau of Alcohol, Tobacco and Firearms, and the Connecticut State Police bomb squad.
"It was an explosive device" that caused the damage, said Mayor John DeStefano Jr.
FBI Special Agent Michael J. Wolf confirmed there was a "potent" device, but said there had been no warning prior to the blast or claim of responsibility afterward.
"There is no indication it was terrorism-related at this time," Wolf said.
The blast occurred just before 5 p.m., hours after President Bush addressed graduates at the U.S. Coast Guard Academy in New London, Conn., some 50 miles away.
While there were no apparent links to international terrorist organizations, the explosion frayed nerves and sent federal agencies ? including the Department of Homeland of Security -- into action because the nation's terror threat level was raised Tuesday to orange, the second-highest level.
Investigators were combing the debris for forensic evidence, Wolf said. Potential suspects were also being sought for questioning, he said.
Yale spokeswoman Karen Peart said it appeared the device "went off in an empty classroom," and damaged that room and an adjacent lounge.
It was unclear how many students were in the building at the time, either studying or taking exams, but those inside fled.
"I sat down to study and there was a very large explosion, which just reverberated," Mike Pyle, a second-year law student, told the New Haven Register. "I looked at my friends, and we realized it was time to get out."
One student, Robert Hoo, said he saw a "huge fireball" come down the hallway and then suddenly disappear.
The law school will be closed Thursday and Friday, but Yale Secretary Linda Lorimer said the annual Class Day on Sunday and undergraduate commencement on Monday would proceed as scheduled. The law school's commencement Monday will be relocated.
It was the second explosion at Yale in a decade. On June 24, 1993, Yale Professor David J. Gelertner was severely injured when a letter bomb sent to him by the Unabomber, Theodore Kaczynski, exploded in his campus office.
As a result, Yale has spent millions upgrading security on campus.
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