Police found 49 mutilated bodies scattered in a pool of blood near the border with the U.S., a region where Mexico's two dominant drug cartels are trying to outdo each other in bloodshed while warring over smuggling routes.
The bodies of 43 men and six women with their heads, hands and feet chopped off were dumped at the entrance to the town of San Juan, on a highway that connects the industrial city of Monterrey with Reynosa, across from McAllen, Texas.
At the spot where authorities discovered the bodies before dawn Sunday, a white stone arch that normally welcomes visitors to the town was spray-painted with "100% Zeta" in black letters - an apparent reference to the fearsome Zetas drug cartel that was founded by deserters from the Mexican army's special forces.
The bodies, some of them in plastic garbage bags, were most likely brought to the spot and dropped from the back of a dump truck, Nuevo Leon state security spokesman Jorge Domene said.
Domene said the dead would be hard to identify because of the lack of heads, hands and feet. The remains were taken to a Monterrey auditorium for DNA tests.
The victims could have been killed as long as two days ago at another location, then transported to San Juan, a town in the municipality of Cadereyta, 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.
Only one couple looking for their missing daughter visited the morgue in Monterrey where autopsies were being performed Sunday, a state police investigator said.
The officer, who spoke on condition of anonymity because he was not authorized to discuss the case, said none of the six female bodies matched the missing daughter's description. He said some of the bodies were badly decomposed and some had their whole arms or lower legs missing.
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 between the Zetas and the powerful Sinaloa Cartel.
Mass body dumpings have increased around Mexico in the last six months of escalating fighting between the Zetas and Sinaloa, which is led by fugitive drug lord Joaquin "El Chapo" Guzman, and its allies, the federal Attorney General's Office said in statement late Sunday.
The two cartels have committed "irrational acts of inhumane and inadmissible violence in their dispute," the office said, reiterating it is offering $2 million rewards for information leading to the arrests of Guzman, Ismael Zambada, another Sinaloa cartel leader, and Zetas' leaders Heriberto Lazacano Lazcano and Miguel Trevino.
Under President Felipe Calderon's nearly six-year offensive against organized crime, the two cartels have emerged as Mexico's two most powerful gangs 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.
"This is the most definitive of all the cartel wars," said Raul Benitez Manaut, a security expert at Mexico's National Autonomous University.
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 government's success in killing or arresting cartel leaders has fractured other once big cartels into weaker, quarreling bands that in many cases are lining up with either the Zetas or Sinaloa. At least one of those 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 on a main boulevard 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 has referred directly to mass killings. All 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.