Was it possible for them to have avoided getting caught? I think so. The reason why they're caught is more of their own faults rather than the capabilities of the FBI to track them down. Of course it is very hard to be extremely careful every step of the way. It might even be next to impossible to avoid committing an act of negligence or carelessness forever. It is hard not to commit a mistake and leave a trace. But it could be done.
If they are not been suspected of suspicious activities leading to the exchanges reports, they are still free till now. But yes, they leave such traces that could help the FBI to detect the couple's fraud activities. Honestly, they are not too smart to hide their doings but they are too bold enough to leave such a trace.Even we think it was a success story for the FBI in hunting criminals like them, keeping anonymous won't easily give them the idea of who are they. But with their simple mistake could somewhat give a hint to the authorities and that is the reason why they end up getting caught.
This reminds me that criminals prefer fiat money to digital money, when Pablo Escobar's time handled a lot of money and the way they counted millions of dollars was by weighing them, just like weighing 1Kg of beef , well, the dollars and that was the way for them to count quickly because it was a lot of money that counting it normally was something almost endless, it took a long time, and like that in physical form, well it was palpable, sure, maybe that's why the great crimes have been done with fiat money than digital money.
and now that crypto does exist or simply talks about digital currencies then they do add up some option but doesnt mean that they would leave out on something that they've been dealing through ages.
Bitcoin is indeed not a safe haven for criminals but the media and government do continue to push it up to be like this.
