Do not need a visa. However, it will be required to present one of the following documents at the airport or port of entry into Mexico:

Valid United States Passport;
Original Birth Certificate and photo official identification (i.e., driver's license);
Certified copy of Certificate of Naturalization and photo official identification (i.e., driver's license);
Voter's Registration Card with photo official identification;
Notarized Affidavit of Birth Facts and a official identification

and what part of mexico u go?