I was investigated once (very minor charge) and I received a letter from the SEC with questions to answer and that was the last I heard of it. The son of one of my elderly clients thought I had put his mother in an investment that was not suitable for her.
When I was the Compliance Officer at Kidder, Peabody, sometimes I would be notified (as a courtesy to the firm) that an investigation was being instigated. If it’s something serious though, they would probably be doing everything in the background until they had enough evidence to make an indictment or file charges. My guess is that any of the scenerios you mentioned might be possible.
So, I think it all depends on the severity of the suspicions/acusations. All I know is when the SEC says "Jump" you don’t even ask how high, you just jump! You don’t fool around with them at all. They carry a lot of weight and power, IMO even more than the FBI.