Being a private citizen is no barrier at all. Here’s a link from the first page of the google search for “civil rights violation” + “double jeopardy”:
A guy is acquitted of murder, the Feds step right in with Violation of Civil Rights, he’s convicted, it’s not double jeopardy because you see it’s the Feds not a state, next case please.
A more fruitful google is for “Lemrick Nelson”, and in fact the Feds got TWO trials to put him away. Does this sound familiar — or predictive?:
“In 1997, following nonstop pressure on the Clinton administration, Nelson was charged in federal court with violating Rosenbaum’s civil rights, under a statute used during the civil rights era to sidestep the racism of Southern white juries.”
http://forward.com/articles/8715/legal-twist-crown-heights-killer-asking-to-get-aw/