Virginia Tech is the largest university in the state of Virginia, with 26,000 students. While there is an on-campus police force, there is no way that they have enough people put armed police immediately on every corner on campus. For big campus-wide events (e.g., football/basketball games), they have to rely heavily on assistance from police forces in neighboring localities. Even in an extreme emergency like today’s, it would take several hours to get enough personnel to campus to implement that kind of security all across the whole campus.