NEW : A message left by a drug gang claims responsibility , the city 's prosecutor says . The nation 's defense ministry was also nearby . NEW : The mayor of Mexico City is in talks with President Obama about sending more troops to fight cartels and stop them from smuggling drugs into America. The heads were found near the border between Mexico City and neighboring Mexico state . Brutal turf battles between drug cartels , common elsewhere in Mexico , are rare in the capital .
NEW : A message left by a drug gang claims responsibility , the city 's prosecutor says . NEW : The mayor of Mexico City is in talks with President Obama about sending more troops to fight cartels and stop them from smuggling drugs into America. The heads were found near the border between Mexico City and neighboring Mexico state . Brutal turf battles between drug cartels , common elsewhere in Mexico , are rare in the capital .