Updated 12:13 p.m. clarified attorney general's ruling

BLACKSBURG, Va. — 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. 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.

Several thousand students attending summer classes, as well as the school's 6,500 employees, were on campus when the alert was issued, Hincker said. Many of the school's 30,000 students are on summer break and will return when the fall semester begins Aug. 22.

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.

Hincker said he was not certain when the lockdown might be lifted.

"That's the $64,000 question," he said. "You get this report of a sighting that someone might have had a weapon. Then you've got this one-square-mile campus, 150 major buildings with several million square feet of space to search."

The school's website was inundated throughout the morning, and school officials said they were bringing additional servers online to deal with the traffic.

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. The university said in a tweet posted just before noon that no other sightings had been reported but asked people to stay inside.

Virginia Tech Police Chief Wendell Flinchum said that the three children were interviewed, and that the information they gave was deemed credible.

The children who made the report were visiting the campus as part of a summer academic program for middle schoolers in Washington, Richard Tagle, CEO of the group Higher Achievement, said in an emailed statement. All the students who were with the group are safe, he said.

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.

"We're in a new era. Obviously this campus experienced something pretty terrible four years ago … regardless of what your intuition and your experience as a public safety officer tells you, you are really forced to issue an alert, and that's where we believe we are right now," said Hincker, the Virginia Tech spokesman.

S. Daniel Carter, director of public policy for Security On Campus, a nonprofit organization that monitors how colleges react to emergencies, said it appeared Virginia Tech responded appropriately. Carter's organization had pressed for an investigation into the school's handling of the 2007 shootings.

"You have to take all of the reports seriously because you cannot take the risk that there's something serious going on and you failed to act," Carter said. "The key is the community was informed so they were able to take steps to protect themselves."

Carter said having various forms of notification — sirens and message boards in addition to text messages and e-mails — are important in instances like Thursday's, when many on campus are there for summer camps or otherwise not registered to receive alerts individually.

Last month, Virginia Attorney General Ken Cuccinelli issued a legal opinion that said public university policies generally can prohibit people from openly carrying firearms in campus buildings and at events. However, such a policy would not apply to someone who had a valid concealed carry permit and carried a concealed firearm.

Federal authorities fined the school in March after ruling that administrators violated campus safety law by waiting too long to notify staff and students about a potential threat after two students were shot to death April 16, 2007, in West Ambler Johnston Hall, 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 to a classroom building where he killed 30 more students and faculty and himself. It was the deadliest mass shooting in modern U.S. history.

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

In 2009, a woman was decapitated while having coffee with a fellow student in a campus café. Police said at the time that officers detained the suspect within minutes of being called. The school said it sent some 30,000 notifications by voicemail, email and text message, though they were not sent as emergency alerts because the suspect was already in custody.

On Thursday, officials said they were looking for a 6-foot-tall white man with light brown hair. Officials said the person was clean-shaven and wearing a blue and white striped shirt, gray shorts and brown sandals. He was described as clean-shaven, according to the university's website.