It took them a while to find the guy. Once they did, though, they found his manifesto and reached the conclusion that he was a racist. He said as much. When the selfies he took were released, the media started to play up the Confederate flag angle.
Something is strange about the Chattanooga case, though. The FBI says it's too soon to tell if he's been "radicalized" but they are treating him as a "homegrown violent extremist." If somebody's a violent extremist, haven't they been radicalized? Either these are very painstakingly nuanced terms, or the authorities are playing word games.
If they can't tell if he's been "radicalized" yet, then aren't they pretty well admitting that this could just be normal behavior for them?