There has been no actual proof presented that I know of. He also probably watched brutal liberal Hollywood movies and played desensitizing video games. Bottom line, who knows?
In addition to symbols on his clothes, you will see ample proof of his far right white supremacist leanings.
But, sadly, I agree with you, oddly. I'm guessing he consumed plenty of John Wayne and Clint Eastwood movies, which glorified gun violence.
But let's be clear. The kid went to an activist black church after confessing to his friends that black people were taking over the world.
Wait 'till the stories about his sympathy with Trayvon Martin come out, which his friends have already reported to the police. Dylann drank of the rightwing paranoia that black thugs were preying upon white neighborhoods.