The blast late Wednesday caused no injuries and limited damage but jangled nerves in a nation on heightened alert for possible terrorist attacks.
Officials reiterated at a midday news conference that there were no apparent links to international terrorist organizations.
"We've pretty much returned to normal with the exception of the crime scene inside the law school," New Haven Mayor John DeStefano Jr. said.
He said all buildings on the Yale campus except the Law School reopened as usual Thursday morning.
"We're getting ready for commencement weekend at Yale," the mayor said. Security will be increased for the events.
"The point that we're at is acknowledging that we had an explosive device," DeStefano said. "We are not aware of any link it has to international terrorism."
State police said investigators are "literally on hand and knee sifting through evidence" in hopes of finding whoever set off the explosion.
Regarding speculation it was a pipe bomb, state police said it has not yet been determined what type of device was used.
The FBI's Joint Terrorism Task Force immediately responded to the scene late Wednesday afternoon along with agents from the U.S. Bureau of Alcohol, Tobacco and Firearms, and the Connecticut State Police bomb squad.
FBI Special Agent Michael J. Wolf confirmed a "potent" device had been used, 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 had been 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.
The explosion damaged an empty classroom and an adjacent lounge, Yale officials said.
It was unclear how many students were in the building at the time, but the explosion sent them fleeing for safety.
"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."
Another 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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