Remember the vast majority of any crime or terrorist action has pre-events leading up to it. For instance a purse snatcher does not just reach out and grab a purse, he takes his time looking for possible police presence, a good quick get away route, somewhere to dump or hand off the purse quickly, all of it. Sometimes in only takes them a few min, to set up for attack, but usually an hour or so. Terrorists can take up to years for the pre-attack planning. If you discover them at any point up to the attack, they will usually break off and look for another target.