@MisuseCase - he's also written a review of Kaczynski's manifesto on Good Reads.. so I'm 100% certain he was already on a list somewhere. All they had to do was look at the people they have data on for making these sorts of social media posts etc, and pick a white guy with dark eyes and strong eyebrows who looks like the pictures they have.
Investigating a crime isn't about solving it, it's about building a brief of evidence that will convict someone.