By Luis Ochoa
CADEREYTA JIMENEZ, Mexico, May 13 (Reuters) - Suspected drug cartel killers in Mexico dumped 49 headless bodies on a highway near the northern city of Monterrey, a sickening atrocity that prompted the government to condemn the "inhuman" violence plaguing the country.
The corpses of 43 men and six women, whose hands and feet had also been cut off, were found in a pile on a highway in the municipality of Cadereyta Jimenez early on Sunday, officials from the state of Nuevo Leon said.
The Nuevo Leon government said the notorious Zetas drug gang had claimed responsibility for the bloodbath, one of the worst to hit Mexico during its struggle against the powerful cartels.
The massacre follows several other mass slayings in Mexico. Many have occurred in the north, where the Zetas have waged a war against rival groups for control of smuggling routes into the United States, the biggest market for illicit drugs.
The Mexican government said in a statement the evidence suggested turf wars between the Zetas and the Sinaloa cartel of Joaquin "Shorty" Guzman were behind the surge in bloodshed.
Condemning the "inhuman episodes of irrational violence" the warring gangs had caused, the government pledged to uphold justice in Mexico with "all of its force and might."
The Zetas gang was founded by deserters from the Mexican army who became enforcers for the Gulf cartel, which once dominated the drug trade in northeastern Mexico. Leaders of the Zetas later split from their employers and the two gangs have since fought for control of trafficking routes.
The Zetas have also been at war with the Sinaloa cartel headed by Guzman, the country's most wanted man.
President Felipe Calderon has staked his reputation on bringing Mexico's drug gangs to heel, sending in the army to fight them shortly after taking power in December 2006.
But the violence has spiraled since, and more than 50,000 people have fallen victim to the conflict, eroding support for Calderon's conservative National Action Party (PAN), which looks likely to lose power in presidential elections on July 1.
A poll published on Sunday showed PAN presidential candidate Josefina Vazquez Mota trailing front-runner Enrique Pena Nieto of the opposition Institutional Revolutionary Party (PRI) by 19 points with just seven weeks to go.
The commercial hub of Monterrey was long a bastion of the PAN, and the local business community has been "livid" about the violence engulfing the city, said George W. Grayson, a Mexico expert at the College of William and Mary in Virginia.
"This puts the final nail in the coffin of the PAN in the presidential contest," he said after the latest atrocity.
Surveys show voters think that the PRI, which ruled Mexico for 71 years until 2000, is more likely to quell the violence. Its long rule was tainted by corruption and critics have accused the PRI of making deals with cartels to maintain order.
Vazquez Mota took a swipe at the PRI after the headless bodies were discovered, suggesting regions governed by the centrist party - which include Nuevo Leon and nearly two-thirds of Mexico's states - had allowed crime to flourish.
"What happened in Cadereyta must be investigated, but what I think it shows, in a terrible and painful way, is the kind of permissiveness seen in these states towards organized crime," she told Reuters in the eastern port of Veracruz on Sunday.
The headless victims have not been identified.
The bodies showed signs of decay, indicating they may have been dead for days, Nuevo Leon Attorney General Adrian de la Garza said. He noted there had been no mass disappearances reported in the state, so the victims could have died elsewhere.
De la Garza said many of the bodies were tattooed, which could offer a clue to their identities. The dead may have been migrants passing through Mexico to the United States, he added. Migrants have been targeted by criminal gangs in the past.
Violent street gangs in Central America like the Maras have distinctive tattoos, though Nuevo Leon public security spokesman Jorge Domene said the victims did not show these markings.
Domene said some had tattoos of Santa Muerte, or "Holy Death" a female skeletal grim reaper venerated by both gangs and some broader, non-criminal sections of Mexican society.
The corpses were taken to Monterrey and authorities said they would perform DNA tests. Thousands of Mexico's drug war victims have never been identified.
SPIRAL OF VIOLENCE
The bloody killings in Nuevo Leon were the worst there since 52 people died in an arson attack on a casino in Monterrey in August. That attack was also blamed on the Zetas.
Monterrey is Mexico's most affluent city and was long seen as a model of economic development in Latin America. But it has been ravaged by the drug war over the last three years.
The horrifying conflict has been marked by an escalation of mass slaughter in recent weeks.
Last Wednesday, 18 people were found decapitated and dismembered near Mexico's second-largest city, Guadalajara.
A week earlier, the bodies of nine people were found hanging from a bridge and 14 others found dismembered in the city of Nuevo Laredo, just across the U.S. border from Laredo in Texas.
Security analyst Alberto Islas said much of the recent spike in violence was the result of fighting over cocaine supplies from South America between the Zetas and the Sinaloa cartel.
Increased pressure on Guzman's operations in Colombia this year had prompted the Sinaloa cartel to buy up a bigger share of cocaine from Peru and Ecuador, squeezing the Zetas' supply and sparking tit-for-tat attacks among the gangs, Islas added.
The fact that state and federal authorities had repeatedly failed to capture and prosecute those responsible for the brutality meant the attacks were only getting worse, he said.
"They're fighting across the whole country with complete impunity," he said. "The government has to send out a very clear signal they will stop the violence and find those responsible."
Late last year, several mass killings took place in the eastern state of Veracruz, which has been ravaged by the Zetas. (Reporting by Michael O'Boyle, Dave Graham, Ioan Grillo, Anahi Rama, Mica Rosenberg and Ana Isabel Martinez; Editing by Kieran Murray and Stacey Joyce)