According to a top secret presentation called 'Tor Stinks' leaked by former NSA contractor Snowden, the US National Security Agency says its agents will "never be able to de-anonymise all Tor users all the time," and that even with manual analysis of their online actions, the NSA admits it can only "de-anonymise a very small fraction of Tor users."
Tor, originally standing for The Onion Router, is a system which can be used to create online anonymity by directing web traffic through a worldwide volunteer network consisting of thousands of relays, concealing the user's location and their browsing history from anyone trying to watch them.
Such anonymity meant users could buy illegal drugs, weapons and even hitmen from the SIlk Road website without fear of being caught.