BLACKSBURG, Va. — Virginia Tech was locked down for several hours today after 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. telling students and employees to stay inside and lock their doors.

Text and phone messages were sent to more than 45,000 subscribed to the school’s alert system, along with an email sent to the entire campus, said school spokesman Mark Owczarski. The school’s outdoor sirens also sounded, he said.

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.

Classes were canceled for the day. Police had received no other reports nor found anyone fitting the description the children gave.