The victims could have been killed as long as two days ago at another location, then transported to San Juan, a town in Cadereyta municipality about 105 miles (175 kilometers) west-southwest of McAllen, Texas, and 75 miles (125 kilometers) southwest of the Roma, Texas, border crossing, state Attorney General Adrian de la Garza said.
De la Garza said he did not rule out the possibility that the victims were U.S.-bound migrants.
But it seemed more likely that the killings were the latest salvo in a gruesome game of tit-for-tat in fighting among brutal drug gangs.
"This is the most definitive of all the cartel wars," said Raul Benitez Manaut, a security expert at Mexico's National Autonomous University.
Mass body dumpings have increased around Mexico the last six months as the fearsome Zetas gang goes head to head with the powerful Sinaloa Cartel, led by fugitive drug lord Joaquin "El Chapo" Guzman.
Under President Felipe Calderon's nearly six-year assault on organized crime, the two cartels have become the largest in the country and are battling over strategic transport routes and territory, including along the northern border with the U.S. and in the Gulf coast state of Veracruz.
In less than a month, the mutilated bodies of 14 men were left in a van in downtown Nuevo Laredo, 23 people were found hanged or decapitated in the same border city and 18 dismembered bodied were left near Mexico's second-largest city, Guadalajara. Nuevo Laredo, like Monterrey, is considered Zeta territory, while Guadalajara has long been controlled by gangs loyal to Sinaloa.
The Zetas are a transient gang without real territory or a secure stream of income, unlike Sinaloa with its lucrative cocaine trade and control of smuggling routes and territory, Benitez said. But the Zetas are heavily armed while Sinaloa has a weak enforcement arm, he said. The Zetas, founded by deserters from Mexico's elite special forces, started out as assassins for the Gulf Cartel before those two gangs had a bloody split in early 2010.
The government's success in killing or arresting cartel leaders has fractured some of the big gangs into weaker, quarreling bands that in many cases are lining up with either the Zetas or Sinaloa. At least one of the two cartels is present in nearly all of Mexico's 32 states.
A year ago this month, more than two dozen people most of them Zetas were killed when they tried to infiltrate the Sinaloa's territory in the Pacific Coast state of Nayarit.
But their war started in earnest last fall in Veracruz, a strategic smuggling state with a giant gulf port.
A drug gang allied with Sinaloa left 35 bodies at a freeway overpass in the city of Veracruz in September, and police found 32 other bodies, apparently killed by the same gang, a few days after that. The goal apparently was to take over territory that had been dominated by the Zetas.
Twenty-six bodies were found in November in Guadalajara, another territory being disputed by the Zetas and Sinaloa.
Drug violence has killed more than 47,500 people since Calderon launched a stepped-up offensive when he took office in December 2006.
Mexico is now in the midst of presidential race to replace Calderon, who by law can't run for re-election. Drug violence seems to be escalating, but none of the major candidates, Enrique Pena Nieto, Josefina Vazquez Mota or Andres Manuel Lopez Obrador, has referred to the body dumpings directly. All three say they will stop the violence and make Mexico a more secure place, but offer few details on how their plans would differ from Calderon's.
Benitez said the wave of violence has nothing to do with the presidential election.
"It has the dynamic of a war between cartels," he said.
Associated Press writer Galia Garcia-Palafox in Mexico City contributed to this report.