Worst mistake was a newbie one - he commingled Silk Road and clean coins in the same wallet. At some point he sent someb legit coins to an exchange (where he was likely KYC'd), and change was sent back to his wallet. After a while he moved the Silk Road coins and the change coming from his clean coins was sent together with the dirty ones.

Law enforcement contacted the exchange and got all the details including his IP address.

Source: https://www.justice.gov/usao-sdny/press-release/file/1549821/download