" Fuselier arrived at Columbine with one assumption: multiple gunmen demanded multiple tactics. Fuselier couldn’t afford to think of his adversaries as a unit. Strategies likely to disarm one shooter could infuriate the other. Mass murderers tended to work alone, but when they did pair up, they rarely chose their mirror image. Fuselier knew he was much more likely to find a pair of opposites holed up in that building. It was entirely possible that there was no single why—and much more likely that he would unravel one motive for Eric, another for Dylan. "

― Dave Cullen , Columbine