A man detonated a homemade bomb outside of an Ariana Grande concert at the 21,000-seat Manchester Arena in England, killing 22 people and injuring dozens more Monday night. The bombing served as a ringing reminder that concert venues can be a vulnerable terrorist target. In the wake of the 2015 shooting at Bataclan music club in Paris, Colorado promoters have bolstered their venues with front-and-center security measures.