All I've heard is naive theory of a lone man doing this, covering all his tracks like a covert agent yet not being one. A brilliant lone cryptography researcher whom no one ever heard of, never published anything before, who managed to obfuscate his activity from all his family and friends.
Nick Szabo and/or Hal Finney is the most credible theory outside of deep state theories. I have heard some pretty outrageous dark horses mooted, like Perelman, which I generally discount out of hand, but there's a pretty strong stylometric case for Szabo+Finney.