I believe this has been a well known underlying problem with Tor for a long time as it only really provides anonymity. If you control enough exit nodes, or alternatively, control the means of transmission in enough places, you can narrow the net and get closer to finding things. At the very least, one could get close enough to allow one to focus more traditional investigatory techniques. The problem is this is a fairly large scale problem. Now any organizations with the capacity to do something like that? I seem to recall lots of speculation around the time Snowden came out.