The bodies were found in Tamaulipas state, a few hundred yards (meters) from its border with Veracruz, according to the Tamaulipas attorney general's office.
The area has been the scene of bloody battles between the Gulf and Zetas cartels, and a pair of banners alluding to a rivalry were found in the truck, the statement from the attorney-general's office said.
On Friday, the attorney general's office in Veracruz said it had found 10 bodies in a different area along the border with Tamaulipas after receiving a tip.
On Thursday, three U.S. citizens traveling to spend the holidays with their relatives in Mexico were among those killed in a spree of shooting attacks on buses. In the spree, a group of gunmen attacked three buses in Veracruz, killing a total of seven passengers.
The Americans killed were a mother and her two daughters who were returning to visit relatives in the region.
The five gunmen who allegedly carried out the attacks were later shot to death by soldiers.
Earlier, the gunmen also shot to death three people and killed a fourth with grenade in the nearby town of El Higo, Veracruz.
The U.S. Consulate General in Matamoros, a Mexican border city north of where the attacks occurred, said urged Americans to "exercise caution" when traveling in Veracruz, and "avoid intercity road travel at night."
Local police in Veracruz have become so corrupt that on Wednesday, the government decided to dissolve the entire police force in the state's largest city, also known as Veracruz, and sent the Navy in to patrol.
Some 800 police officers and 300 administrative employees were laid off. Officials said they can apply for jobs in a state police force, but must meet stricter standards.
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