A U.S. university that was the site of a mass shooting in 2007 has been on lockdown for hours, after three youths reported seeing a man who was carrying what they said might have been a gun.

Virginia Tech authorities said Thursday that they issued orders for people to stay inside with doors locked.

Three young people attending a summer camp at the university reported the man early Thursday, saying he appeared to have what might have been a gun hidden under a cloth. The report triggered an immediate search by police and campus security.

Three hours after the lockdown began, police issued a message saying there have yet to be any more sightings of the person. But the university has announced the lockdown will continue, and classes are canceled for the rest of the day. Authorities have released a composite sketch of the suspect.

A school spokesman told reporters it is important to be cautious, given the incident in 2007, in which a lone shooter killed 32 people and then himself.

Earlier this year, the university was fined by federal authorities for waiting too long to notify staff and students during the rampage.