A suspected bomb has exploded at the prestigious Yale University in the US state of Connecticut.
Buildings were evacuated after the explosion
Smoke could be seen rising from the law school building in the city centre of New Haven shortly after the blast at about 1600 local time (2000 GMT).
It was not immediately clear what caused the explosion inside a general classroom.
But New Haven Mayor John DeStefano said he had been told it may have been an explosive device.
He said there were no reports of injuries.
Earlier on Wednesday the US Government raised the country's level of alert to orange, the second highest, following renewed threats before a long holiday weekend.
Yale law student Bob Hoo who was in a corridor on the ground floor said: "I saw a huge fireball come out to the middle of the hallway."
The blast sent debris flying across campus and the building itself was damaged.
An FBI spokesman said members of the agency's Joint Terrorism Task Force had been sent to the scene as a precaution, local television news reported.
Local authorities are still searching the building for possible secondary devices and the area has been evacuated.
The incident came only hours after US President George W Bush - a Yale graduate - visited Connecticut to speak at the US Coast Guard Academy.
In June 1993 a mail bomb sent by the so-called Unabomber seriously injured a Yale professor.
The bomber, Theodore Kaczynski, was jailed in 1998 for a series of attacks, which left three people dead and scores hurt.