Silk Road is (was?) an anonymous website where you could buy drugs. It was only reachable through an anonymous computer network called Tor, so, in theory, untraceable.
As it turns out, they were able to tie the work back to the guy running it and build a case against him over time. Apparently he was doing $1.2B/year in revenues, $80M profit, and contracted out at least two hits on people, though it is unclear if the hits actually happened.