On or about July 10, 2013, CBP [Customs and Border Patrol] intercepted a package from the mail inbound from Canada as part of a routine border search. The package was found to contain nine counterfeit identity documents."Routine border search" is one of the techniques taught by the "Special Operations Devision" to hide the source of unconstitutionally obtained information. As documented in the Reuters article, when the NSA or FBI obtains unconstitutional evidence against American citizens, they tell border agents what to look for when things cross the borders.
As I've written about recently, the Tor network overwhelmingly uses 1024 bit keys, which everyone believes can be routinely cracked by the NSA. Moreover, the Silk Road has been probably the #1 target the NSA would want to crack. It's possible the service had been compromised for some time, and agents were simply waiting for some trigger (such as forged IDs crossing the border) to start the official investigation.
My point isn't whether this is true in this case, only that's it's true in other cases. It's impossible for a citizen to challenge that "routine border search" in order to discover whether it was truly routine. It's impossible for the defendant to challenge the constitutionality of the evidence against him.
Certainly, Silk Road is guilty of crime, but here's the thing: we now live in a police state, with secret police, that we are powerless to challenge.
Update: On twitter and elsewhere some people have challenged whether the investigation actually started with the border control agents. It's a fair criticism, so I added the word "appear to have" started with that incident. I say it started at this point because it's the first piece of evidence that could not have been obtained by collecting old log files. In other words, the Stack Exchange post was from a year earlier, but the FBI got it from Stack Exchange log files after they started investigating Ulbricht. We won't know the actual timeline until the trial starts in about 2 years from now.
Also, by "investigation" I mean "investigation into Ulbricht" in particular. The investigation into Silk Road doubtlessly started years ago.
Update: Popehat has an excellent discussion of how the investigation naming Ulbricht started earlier, though it raises even more questions pointing "parallel construction" -- of using NSA-style evidence and hiding it.