Well, each Tor relay connects to many other Tor relays. But one could use traffic correlation to at least somewhat focus the search back through the Tor network. However, Tor circuits don't last very long, normally on the order of minutes, so the adversary would need to work very quickly. Indeed, they would need to have all the intercepts set up in advance. Still, there are only a few thousand Tor relays, and they're all (except for bridges) listed publicly. So it's doable for at least some global adversaries. The key limitation, I suspect, is false positives in traffic correlation. See The 23rd Raccoon  How I Learned to Stop Ph34ring NSA and Love the Base Rate Fallacy at http://archives.seul.org/or/dev/Sep-2008/msg00016.html.