After three days, an enigmatic portrait emerged of the 20-year-old man who came close to killing former President Donald Trump with a high-velocity bullet: He was an intelligent loner with few friends, an apparently thin social media footprint and no hints of strong political beliefs that would suggest a motive for an attempted assassination. Even after the FBI cracked into Thomas Matthew Crooks’ cellphone, scoured his computer, home and car, and interviewed more than 100 people, the...