From your link:
Garcia had been living in some form of transient lodging, according to the law enforcement source. Law enforcement sources briefed on the investigation told ABC News that preliminary information developed in the probe indicates that the shooter was in the U.S. Army in 2008 and was "removed due to mental health concerns." In addition to the insignia on the shooter's equipment that suggested a right-wing extremist ideology, investigators have found social media accounts connected to the shooter that reveal hundreds of postings and images, including writings with racially or ethnically motivated violent extremist rhetoric, neo-Nazi material and material espousing the supremacy of the white race. None of the subject's postings analyzed to date were liked or shared by other users nor were there any public comments. The shooter's account did not contain any friends or associates that were publicly visible.
Okay so, in 2008 he would have been 18. Since he's 33 now, he's either hidden his mental health issues from everyone for the last 15 years, or more likely, everyone that knows him knows he has mental health issues.
Also, his social media accounts as reported, seem pretty weird. Hundreds of posts and images, yet NONE of them were liked or shared by anyone, and zero friends visible? I'm having a hard time with that one. Even the creepiest of creepy accounts has people liking and sharing their stuff. Yet this guy had zero likes, shares or comments?