BLACKSBURG, Virginia (AP) -- Virginia Tech was locked down Thursday when three children attending a summer camp said they saw a man holding what looked like a gun on the campus where a 2007 massacre left 33 people dead.

The university issued an alert on its website at 9:37 a.m. local time. Thursday telling students and employees to stay inside and lock their doors. University spokesman Larry Hincker said during a news conference later in the morning that the campus alert remained in effect and that people should stay indoors until further notice.

The university posted the alert on its website and its official Twitter account. The Roanoke Times also reported that the university sounded its emergency sirens and issued an emergency alert by phone and email.

The campus-wide alert was later lifted and students and staff were told to resume their normal activities, according to an email sent at 2:42 p.m. The email said there would be a large police presence on campus throughout the day.

The school posted an update on its website around 1 p.m., saying authorities were combing through buildings on campus. Classes were canceled for the day, and the school said searching the buildings would be a long process. A composite sketch was posted on the school's website. People on campus were told to remain indoors.

The children told police they saw the man quickly walking toward the volleyball courts, carrying what might have been a handgun covered by some type of cloth. State and local police swarmed the area but said they could not find a gunman matching their description.

An alert on the school's website said the gunman was reported near Dietrick Hall, a three-story dining facility steps away from the dorm where the first shootings took place in the 2007 rampage. A student from South Korea, Seung-Hui Cho, killed 30 more students and faculty and himself. It was the deadliest mass shooting in modern U.S. history.