I can't believe some of the terrible advice and just plain trolling posted here so far

You are sitting on tremendously valuable information for the people trying to save this endangered species. I would ask that you not release them on your own until the identity can be confirmed by a recognized expert because you said you caught them in a location where they are believed extirpated. People working with the species need to know that they can be found there.
I don't know what the actual risk is of introducing aquarium-borne disease, but you probably shouldn't release them on your own until you do know.
There are several ways to protect yourself legally, since you really are trying to do the right thing here (and thank you for posting!). The best is to contact one of the people working with the minnows. If that proves difficult, I would think nearly any professor of biology at the local university would be interested and willing to help. A quick email asking to annonymously deliver the fish would be one way to go. Maybe mention that they could publish the finding of the species in a new location.
I would really expect that you'd be fine contacting the game warden and explaining the situation. Once they understand that you're trying to make it right, and you came to them rather than getting caught, I wouldn't expect a ticket.