At a crowded tourist site, a young man in a yellow T-shirt angles for a spot on a bench. He sits, removes his backpack, and places it on the ground. After riffling through a blue plastic shopping bag, he walks away, leaving the backpack behind. A few minutes later, a bomb explodes. Twenty people die.

All that’s known about the young man in the yellow T-shirt is contained in a snippet of dim, grainy footage from a CCTV camera. Would anyone recognize his face?