1. High profile event (Federal building, unique victims, etc)
The Boston marathon, which is always held on Patriots Day in Boston, MA, is the Super Bowl equivalent for runners; All participants must submit qualifying times from previous races and the most elite runners from around the world fly in to compete.
2. Was there a scheduled drill?
(Examples in history: 7/7, The Madrid Bombing, Gladio, . Government always has drills in case the compartmentalized group of mercenaries that are caring it out, while wearing police or FBI uniforms get caught, they can say “OH it was just part of the drill”) Bomb sniffing dogs were at start and finish lines, reported UM coach.