Just before a gunman opened fire on former President Donald Trump at a rally in Butler, Pa., law enforcement was on the verge of a major breakthrough. They had spotted the suspicious individual, Thomas Matthew Crooks, acting oddly but lost track of him. Despite having a clear shot, Crooks managed to climb onto a warehouse roof outside the security perimeter, from where he fired at Trump and killed a rally attendee. The Secret Service, who had been warned about Crooks but didn't act on it, now f...