How did they find out he originally posted under his real name? They must have known that was his profile, and then SO handed over the data proving it?
The SO account was later changed from altoid to frosty. The email address used to register the SO account was rossulbricht@gmail.com.
Also when the FBI imaged the Silk Road server, the username was "frosty". There were just so many links going back to him :-/
There have been long articles about the Silk Road and its demise, the Wired ones have a lot of details including what I mentioned above. Part 1 is here: https://www.wired.com/2015/04/silk-road-1/
Yes, the DOJ subpoenaed Stack Overflow as part of the investigation. It's pretty standard.
Normally the DOJ gets access to all the emails of the target of the investigation, then from there they look through the emails and subpoena any companies that might hold additional information - such as Stack Overflow.
it's a good question, and i could only speculate what sleuthing led them to ask SO for information about that account, but yes, they sent an info request to SO, who complied.
2. Forensic testimony in the complaint asserted Silk Road used this method and in fact used code identical to that in the answer.
3. Silk Road server encryption was signed with Frosty@Frosty.
#2 and #3 were evidentiary, but #1 is what tied everything to a real person's name.