I've been wondering about this. Afaik Ross Ulbricht was caught due to shitty opsec. Was that a case of the NSA having that capability and not sharing it with the FBI?
Ulbricht was first connected to "Dread Pirate Roberts" by Gary Alford, an Internal Revenue Service investigator working with the U.S. Drug Enforcement Administration on the Silk Road case, in mid-2013.The connection was made by linking the username "altoid", used during Silk Road's early days to announce the website, and a forum post in which Ulbricht, posting under the nickname "altoid", asked for programming help and gave his email address, which contained his full name.
