How did they manage to get access to letters of guarantee? Weren't they supposed to be deleted 24 hours after transaction got confirmed?
The investigation started last year, I'm guessing like with some darknet markets, the Dutch police decided they'll run the service themselves for a while to glean some information from people using the service. Or they managed to locate bestmixer's servers and install a backdoor.
This is a real possibility - and makes you wonder if Chipmixer isn't already, or hasn't always been ran by some goverment agency.