I suspect he got the attention of the FBI by being publicly vocal about his hatred of police but was arrested for having a gun when he was prohibited because of his conviction for domestic assault.
The FBI itself admitted that nothing he wrote could be characterized as a threat. However vitriolic, non-threatening speech should never, in a free society, trigger law enforcement action. And note that the gun-possession charge was evidently flimsy enough that a judge dismissed it before trial.
Thank you for saving me from going on an anti-free speech tirade. And now I see the article has been flagged, presumably because the headline is misleading.