It's why "lone wolves" keep LEOs up at night. And right now, I'm hoping and praying Al Qaeda and its awful brethren develop a sever case of illiteracy.
PS:
Re: No mental illness that we know of. The shooter's dad was called a psychopath, but that may be because most people don't tend to think of "bank robber" as a legitimate occupation. I don't know if the dad was ever seen by mental health experts or not. But some mental illnesses are hereditary.
And now I'm arm-chair analyzing.