BLACKSBURG, Va. - (AP) - Virginia Tech was locked down for several hours Thursday 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 tothe school's alert system, along with an email sent to the entirecampus, said school spokesman Mark Owczarski. The school's outdoorsirens also sounded, he said.The campus-wide alert was later lifted and students and staffwere told to resume their normal activities, according to an emailsent at 2:42 p.m. The email said there would be a large policepresence on campus throughout the day.

Classes were canceled for the day. Police had received no otherreports nor found anyone fitting the description the children gave.A composite sketch was posted on the school's website, and officershad scoured the campus for any sign of the possible gunman.Several thousand students attending summer classes, as well asthe school's 6,500 employees, were on campus when the alert wasissued, said University spokesman Larry Hincker. Many of theschool's 30,000 students are on summer break and will return whenthe fall semester begins Aug. 22.Maddie Potter, a 19-year-old rising sophomore from VirginiaBeach, said she was working on a class project inside Burchard Hallwhen a friend received a text message from the school at 9:41 a.m.Soon after, staff locked the doors and turned off the lights.Potter, an interior design major, said she was still holed up ina wood shop inside the building Thursday afternoon. She said thingshad calmed down since the alert went out.

"I was pretty anxious. We had family friends who were up herewhen the shooting took place in 2007, so it was kind of surreal,"she said. "I had my phone with me and I called both my parents."

The children told police they saw the man quickly walking towardthe volleyball courts, carrying what might have been a handguncovered by some type of cloth.

The children who made the report were visiting the campus aspart of a summer academic program for middle schoolers inWashington, Richard Tagle, CEO of the group Higher Achievement,said in an emailed statement. All the students who were with thegroup are safe, he said.An alert on the school's website said the gunman was reportednear Dietrick Hall, a three-story dining facility steps away fromthe dorm where the first shootings took place in the 2007 rampage."We're in a new era. Obviously this campus experiencedsomething pretty terrible four years ago ... regardless of whatyour intuition and your experience as a public safety officer tellsyou, you are really forced to issue an alert, and that's where webelieve we are right now," Hincker said during a morning newsconference.S. Daniel Carter, director of public policy for Security OnCampus, a nonprofit organization that monitors how colleges reactto emergencies, said it appeared Virginia Tech respondedappropriately. Carter's organization had pressed for aninvestigation into the school's handling of the 2007 shootings.