What you do is get a warrant and attack one endpoint or the other. But that involves real police work, and these jokers are allergic.
How many crime 'rings' have been undone because one person came to the attention of the police, and that same one person , despite having the best tools and training etc, kept easily locatable contact lists and other stuff in convenient locations on their computer or around their home?
I can't recall a single actual case where listening to every conversation and breaking crypto would've prevented a crime, but I can think of many where 'stupid is as stupid does' ended a "vast criminal empire".
 This is why I'd never be a criminal mastermind, I draw way too much attention publicly and probably exist one every watch list ever conceived. Probably makes me a good waste of resources as well, as while they again pore over all of my El Reg posts in case there really is something of interest after all, someone else is being ignored who perhaps warrants a bit of extra attention.
 I see I'm far from the only one to have pondered if it's 'pore' or 'pour', and duckduck'ed to confirm.