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.
It is possible. They can just leave the place where they currently leave or change details, everything from their name to emails and the device that use but I think they never did that . they got comfortable and think they will never get caught because the transaction done was online plus it was already a long time when the hack happened but i dont under estimate the fbi . they can hire professionals in the i.t field to do the work . it is true what they say that bitcoin is really not a safe haven not only for the non criminals or the the normal investors because it has risk and more over to the criminals because they could possibly be traced .