(CNN) -- Authorities have opened a murder investigation after the discovery of 13 bodies Sunday in an abandoned truck in the Mexican state of Tamaulipas, the attorney general's office reported.
They were discovered near the border with Veracruz state during routine patrols by national defense forces.
"It's presumed that the case is related to illegal organized crime groups because of a couple of posters that were found in the truck alluding to rivalries between groups of organized crime," said Tamaulipas Attorney General Bolivar Hernandez Garza.
The incident was believed to be tied to a pair of recent attacks in the area that killed 21 people, including three Americans, but the attorney general's office didn't explain how the events may have been connected.
An American consulate general statement warned American citizens to "exercise caution and remain vigilant when traveling in the state of Veracruz. American citizens residing in, visiting or traveling through the northern portion of Veracruz that borders Tamaulipas and San Luis Potosi should maintain a heightened sense of alert." Inter-city road travel at night should be avoided, the consulate general said.
CNN's Esprit Smith contributed to this report.