Spider the web and then correlate less-anonymous writings with the stuff the ringleaders are saying on the Darknet.
As part of another gig I worked on ways to track people, or more commonly to work out if two distinct users were actually the same human, and I'm almost salivating at the thought of all the potential ways authorities could bring some of these people to justice.
Don't know if it resulted in someone being caught by the police or not, but definitely subtler methods such as statistical analysis of writing style are not the only way to get a lead from those forums.
I urge caution against amateur investigations, but it'd be interesting to see what software tools could be written to help law enforcement do a better job.
Statistical analysis of text
"social web"
etc.
You'd have missed out on the opportunity to work with Feynman if you'd declined work at the Manhattan Project.
To use the good old reductio ad absurdum, would you be willing to kill a random stranger if it meant you would get to work with Albert Einstein?
So yeah, you'd need a huge amount of certainty that this was "the guy", and, frankly, storing enough data about individual writing styles for comparison means it's highly unlikely (right now) that this would be possible to do from a technical perspective, and potentially ethically dubious even if the technology were there - what constitutes "public data" on the internet from a privacy and data protection perspective?
And then again I am unsure if simply visiting and contributing to such a forum could get you in any trouble, legally.