information pulled from the shroud of government secrecy has fed conspiracy theorists who believe oswald did not act alone and may have been backed by cuba, russia, the mafia -- >> and karma. i saw a flash of light in the bushes. and then shots rang out. >> reporter: and these latest papers are fascinating serious historians, too. even when they aren't all about oswald. >> i learned some things today, more details about how the mafia were used in an attempt to kill castro. now i sort of understand much better the technique that would have been involved and why it was a serious effort, which did not actually succeed as we know. >> reporter: that sort of information is precisely what intelligence agents are cagey about. they don't want their methods or their contacts known, even decades later after this far-ranging investigation. and that's why about 10,000 documents remain either heavily redacted or entirely off limits.