actually I just saw it on a documentary. they may be using anonymous bitcoin transactions, so they can make transactions for criminals freely. but for now, it may be tracked if there is no mixer inside.
Bitcoin can never be 100% anonymous, even if you use a coin-mixer. Remember the Sheep Marketplace robbery? The thief (Thomas Jiřikovský) was caught despite using more than one mixer to hide his transactions. And recently the cops in Denmark arrested a few drug dealers after tracking their Bitcoin transactions.