MEXICO CITY, Dec. 26 (UPI) -- An investigation was opened in Mexico's Tamaulipas state Monday, where authorities found 13 bodies in an abandoned truck, the attorney general office said.
The bodies were discovered Sunday near the border with Veracruz during a patrol by national defense forces, CNN reported Monday.
"It's presumed that the case is related to illegal organized crime groups" because of some items found in the truck, said Tamaulipas Attorney General Bolivar Hernandez Garza.
Officials said the incident could be tied to two recent attacks in the area in which 21 people, including three Americans, were killed, but the attorney general's office didn't explain how the incidents were related, CNN said.
The area where the bodies were found is part of the territory disputed by rival Los Zetas and the Gulf drug cartels, El Universal reported.
On Saturday in the Veracruz border community of Tampico Alto near Tamaulipas state, authorities found 10 bodies. On Friday, gunmen opened fire on a passenger bus, killing 11 people.
A statement from the U.S. consulate general urged American citizens to exercise caution and "remain vigilant," warning that travel between cities at night should be avoided.