BLACKSBURG, VA - (AP) - Virginia Tech was locked down for severalhours Thursday after three children attending a summer camp saidthey saw a man holding what looked like a gun on the campus where a2007 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.

"You have to take all of the reports seriously because youcannot take the risk that there's something serious going on andyou failed to act," Carter said. "The key is the community wasinformed so they were able to take steps to protect themselves."

Carter said having various forms of notification - sirens andmessage boards in addition to text messages and e-mails - areimportant in instances like Thursday's, when many on campus arethere for summer camps or otherwise not registered to receivealerts individually.

Virginia Secretary of Public Safety Marla Decker said she wasglad the children reported what they saw.

"We'd rather have a report come to us, investigate it and laterin the day say there was nothing to it," she said.

Rachel Larson, a 22-year-old English and communications studentfrom Winchester, Va., got a text message alert at her off-campusapartment.

"At first I was a little confused because Virginia Tech - eversince 4/16 - we've been so paranoid. We hear about everything thatgoes on on campus, which is good, but sometimes people freak outwhen it's a false alarm," she said. "Then I realized my boyfriendwas on campus and I started to freak out a little bit."

Larson said her boyfriend was locked down in the student unionfor several hours but eventually was allowed to leave.

"In the morning everyone was kind of concerned, but as the daywent on we kind of realized it's not anything. No one is reallythat worried anymore," she said.

Last month, Virginia Attorney General Ken Cuccinelli issued alegal opinion that said public university policies generally canprohibit people from openly carrying firearms in campus buildingsand at events. However, such a policy would not apply to someonewho had a valid concealed carry permit and carried a concealedfirearm.

Federal authorities fined the school in March after ruling thatadministrators violated campus safety law by waiting too long tonotify staff and students about a potential threat after twostudents were shot to death April 16, 2007, in West Ambler JohnstonHall, a dorm near the dining facility.

An email alert went out more than two hours later that day,about the time student Seung-Hui Cho was chaining shut the doors toa classroom building where he killed 30 more students and facultyand himself. It was the deadliest mass shooting in modern U.S.history.

The school's alert system also was activated in 2008, when anexploded cartridge from a nail gun produced sounds similar togunfire near a campus dormitory. It was the first time the systemwas activated after the 2007 massacre. After the shootings,Virginia Tech started using text messages and other methods besidesemails to warn students of danger.

In 2009, a woman was decapitated while having coffee with afellow student in a campus cafDe. Police said at the time thatofficers detained the suspect within minutes of being called. Theschool said it sent some 30,000 notifications by voicemail, emailand text message, though they were not sent as emergency alertsbecause the suspect was already in custody.

On Thursday, officials said they were looking for a 6-foot-tallwhite man with light brown hair. Officials said the person wasclean-shaven and wearing a blue and white striped shirt, grayshorts and brown sandals.