evidence: Even after the FBI cracked into Thomas Matthew Crooks’ cellphone, scoured his computer, home and car, and interviewed more than 100 people, the mystery of why he opened fire on Trump’s rally Saturday, wounding the GOP nominee, remained as elusive as the moment it happened. [...] The day before the shooting, Thomas Crooks went to the sportsman’s club and practiced on the rifle range, according to a federal intelligence briefing obtained by the AP. On the day of the attack, he purchased 50 rounds of 5.56mm ammo for his rifle from a local gun shop and drove alone to Butler, Pennsylvania, the site of the Trump rally. [...] Witnesses and law enforcement officials say Crooks walked around for at least a half-hour before climbing onto the roof of a building adjacent to the Butler Farm Show grounds, where Trump was speaking. As spectators screamed for police to respond, Crooks opened fire, letting loose two quick bursts. A Secret Service counter sniper fired back within about 15 seconds, killing Crooks with a shot to the head.