Via Instapundit we learn this interesting tidbit about active shooter incidents:

More than two-thirds of the incidents analyzed by the FBI happened at businesses or schools. In many cases, they happened so quickly that the shooting was over before police arrived. Of 64 incidents analyzed, 44 were over in five minutes or less. Of those, 23 ended in two minutes or less.

As As Glenn Reynolds correctly notes, "This is why you need to have armed individuals dispersed throughout the population - the only people in a position to respond quickly enough are the people who are already there."