A contagious shooting is a sociological phenomenon observed in military and police personnel in the United States, in which one person firing on a target can induce others to begin shooting. Often the subsequent shooters will not know why they are firing. O'Donnell partially reinforces this, saying that in classic cases involving contagious shooting, "a gun was shot before any officers fired," and thus "the officers involved began shooting because of fear or because of the sound of a colleague firing."