In 2013, after threatening to shoot his resident counselor at Rutgers University, the then-19-year-old Yoo was committed to a New Jersey psychiatric hospital, where he was diagnosed with “aggressive homicidal ideation and explosive personality disorder.” A year later a friend was alarmed enough at Yoo’s behavior at a shooting range to alert police. He “would shoot at the head of the target every time, even though range policy specifically prohibited such contact,” he reported. Even after warnings, Yoo “continued to shoot at the head.”

Yoo was admitted to a psychiatric clinic again in 2015 “after repetitive disruptions [and] homicidal threats.” Several colleges banned him from their campuses. He was twice rejected for military service because of his unstable mental health. Online, he became known as the “Asian Nazi” for his racist rants and threats of violence against blacks and Jews.

Yet the Texas Department of Public Safety issued Yoo a license to carry a handgun.