Authorities issued a lockdown on Thursday at the campus of Virginia Tech, site of a 2007 mass shooting that killed 32 people, after a man suspected of carrying a gun was seen on campus, the school said.

An alert on the school's website said: "Person with a gun reported near Dietrick (a dining hall). Stay inside. Secure doors. Emergency personnel responding. Call 911 for help."

Police and school officials said three youths attending a Higher Achievement camp on campus reported seeing a white man carrying what appeared to be a gun covered with a cloth.

Wendell Flinchum, the chief of Virginia Tech's police force, said in a televised news conference that police were still looking and wanted everyone on campus to stay indoors.

Nearly three hours after the initial sighting, the school issued an update on its website telling people to remain indoors but said there had been no other sightings of the man.

Activity at the school, which is currently being used to house dozens of academic and sports camps for children over the summer vacation, ground to a halt after the lockdown and doors on campus were locked to keep people inside.

Police described the suspect as being 6 feet tall with light brown hair and wearing a blue and white striped shirt, gray shorts and brown sandals.

Virginia Tech, formally known as Virginia Polytechnic Institute and State University, was criticized for not reacting quickly enough when gunman Seung-Hui Cho killed 32 people and then himself on the campus in April 2007.

It responded quickly this time.

After sending out an initial warning on its website, the school continued to update people on campus and the city of Blacksburg kept its citizens apprised of the situation through its own website.

"We're in a new era. Obviously, this campus experienced something pretty terrible four years ago," school spokesman Larry Hincker said at the news conference.

Local and state police were combing the campus. An FBI spokeswoman in Richmond said agents were going to the scene.