evidence: A sniper cut down 20-year-old Thomas Matthew Crooks within seconds of him firing an AR-style rifle toward the former president, but it was too late. Now investigators are trying to painstakingly piece together how an armed man with no military background managed to reach high ground and get the jump on teams of Secret Service agents. [...] The FBI said it was investigating the attack as a potential act of domestic terrorism, but the agency had not identified a clear ideological motive. The FBI believes Crooks, who had bomb-making materials in the car he drove to the rally, acted alone. [...] The roof where Crooks lay was an estimated 147 yards (135 meters) from where Trump was speaking, a distance from which a decent marksman could reasonably hit a human-sized target. That is a distance at which U.S. Army recruits must hit a scaled human-sized silhouette to qualify with the M-16 rifle.