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.
I think its already happen a long time, in the more deep web, you can easily found that this bitcoin is become the main currency for crime transaction.
Like, trade for gun, drugs, even human.
This is insane but its real
It could really in illegal doing because every transactions done without knowing who that person is. It hides the identity of the person who use it that is why it is hard to track who made the crime. The government had nothing to do with it but if happen in particular country, that country may ban to use bitcoin anymore. Because there are lot of profit from investing in bitcoin some user abusively used it in wrong way.