Officials at a U.S. university that was the site of a mass shooting in 2007 have lifted an hours-long lockdown after police found no trace of a reported gunman.

Virginia Tech Thursday sent out an E-mail message instructing students and staff to resume their normal activities. It also said a large police presence would remain on school grounds.

Officials first told people to stay inside with doors locked after three youths reported seeing a man who was carrying what they said might have been a gun.

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

Authorities released a composite sketch of the suspect based on the alleged sighting.

A university spokesman told reporters the school wanted 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.