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.
My guess is that there's people out there trying to decipher some sort of code to try and track the criminals. However wouldn't they just be able to do this through their IP and active computer and device monitoring anyway?
I don't know how FBI works but remember on how did someone tracked silkroad and shuts it down. I watched a video related to it that the owner of that marketplace is also here in forum and the simple guy from the FBI did researched all of the things throughout the entire forum since bitcoin is the currency they used there. I'm not that much knowledgeable when it comes to these things, I just watched it.
That video that you are trying to say is a very great work by the FBI personnel because he just roam around and keep joining forums and doing a lot of research to finally find out who is behind the marketplace of silkroad until he find out the IP address and locate it and get the person behind silkroad and that is not the only time where bitcoin is being used in crimes because maybe for simple or small transactions, bitcoin is being used illegally.