I wouldn't contact any state agency until you know what it is. If it is endangered, they will press charges immediately and give you no recourse. Last thing you want is fish & game beating down your door. If its not a native species, chances are they won't have a clue about it at all. There are several other species of tortoise considered 'desert' tortoises that aren't endangered - so I can't really say without seeing it myself. I'm just real wary, animal agencies are not kind at all to people who they consider to be harming
endangered species. Intentional or not.
If you do determine that it is an endangered species, call your state wildlife office (fish & game, whatever it is called where you are) and ask for a licensed state wildlife rehabilitator that deals with the species. Then take it to them. It may be hard to give up a pet, but thats the only 'legal' method of turning over an endangered species.
Rav