There are cryptocurrencies like monero whose primary purpose is to facilitate transactions between wallets that cannot be observed (I think).
If they've traded into that currency somewhere, how does one know where that money pops back up - on however many exchanges, under however many identities, in however many amounts, over whatever period of time they drip it back in?
I'm reminded of a paper I read a while back about deanonymizing VPN traffic if you have sufficient observability of nodes in the overall network and something else I can't remember at the moment.
Seems different though. The time they could take to drip money back in to the visible network (for conversion to fiat or appreciation in a "visible" coin) feels like a factor.
edit - heh, just now seeing the article you posted about the FBI's team explicitly mentions a case like this with Monero.