Officials at Virginia Tech university, scene of the worst school shooting in US history, on Thursday locked down the campus after three youths said they saw a white man possibly carrying a handgun. So far, police have not found anyone matching the description given by the youths, who were attending a camp at the university in Blacksburg, Virginia, but were still patrolling the campus in search of the man. \"People on campus stay indoors until further notice,\" the university said in a bulletin on its website. The university said that at 9:09 am (1309 GMT), the three youths saw a \"white male, six feet tall, with light brown hair outside of New Residence Hall East holding what may have been a handgun.\" \"They say the weapon was covered by a cloth or covering of some sort,\" the statement said, adding that the suspect was wearing a shirt with blue and white vertical stripes, gray shorts and brown sandals. \"Officers responded immediately to the area but found no one matching the description. Police have encountered no other witnesses reporting this individual or anything suspicious,\" the university said. Local television station WSLS reported Thursday that the campus siren system had been activated. The fall semester has not yet begun, though summer programs are in session. In 2007, Cho Seung-Hui, a 23-year-old Virginia Tech student born in South Korea, shot to death 32 fellow classmates and teachers before killing himself. The university in Blacksburg, Virginia was fined $55,000 in relation to the April 16, 2007 incident for failing to alert students in time that a gunman was loose on the campus.