The article also states

Deschutes County District Attorney Steve Gunnels said investigators had recovered journal entries with Garner’s belongings that indicated he was not planning to target specific people, but had a “generalized anger.” Court charging documents list potential victims in the case as “spectator” and “belayer.”

(...)

The planned attack appeared to target the Smith Rock Craggin Classic, an event hosted by the American Alpine Club and described as a “3-day grassroots climbing festival.” The American Alpine Club thanked law enforcement in a social media post, and said they became aware of the threat Wednesday, a day before Garner’s arrest. The group did not immediately respond to a request for comment.

Gunnels, the Deschutes County prosecutor, said Central Oregon law enforcement became aware of Garner after Portland police received tips about the potential shooting.

Orwellian... no?

It's laudable that they intercepted the guy, but I have some minor concerns about exactly how they knew what he was thinking, and some serious concerns about the fact that they intend to prosecute him for thinking it.

May as well just overtly prosecute it as thoughtcrime and send the dude off to the Big Brother School of Rehabilitation for some programming...