Four years after the nation's deadliest school shooting, a report of another armed man on the grounds of Virginia Tech sparked an all-day manhunt Thursday.

A campus-wide lockdown was instituted just 28 minutes after three teens attending a summer program at Virginia Tech reported seeing a man carrying a weapon. While officers searched some 150 buildings for the suspect, the university activated a new emergency alert system put in place after the 2007 rampage that claimed the lives of 33 students.

Students and staff received warnings about the situation by phone, email and text message. Alerts were also posted on the university's website, Facebook and Twitter accounts and on 600 lighted boards installed in school buildings.

Although no gunman was found, police later issued a composite sketch of a person of interest described as a white male, six-feet tall, with close-cropped light brown hair. He was last seen wearing a blue-and-white striped shirt, gray shorts and brown sandals.

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