More than 24 hours after the gruesome discovery, officials had yet to identify any of the mutilated corpses found near the northern industrial city of Monterrey. None of the bodies examined so far showed signs of gunshots, Nuevo Leon state security spokesman Jorge Domene told Milenio television.

Though it was unclear who the victims were, it was the fourth massacre in a month. Mexico’s interior secretary, Alejandro Poire, said Monday that all those incidents resulted from the fight between the Zetas gang and the Sinaloa Cartel, which have emerged in the last year as the two main forces in Mexican drug-trafficking and other organized crime.

Some victims in earlier body dumps have turned out to be bakers, brick layers, even students — anyone who could be snatched off the streets in mass killings that one captured gang member said were designed to "cause terror."

Poire would not respond directly when asked by The Associated Press if innocents have increasingly become targets.

"We don’t have proper identification of the dead," he said. "We have to leave that to the investigation."

"We have to look deeper … to know the motives or who could have been the victims of violence," Poire added.

The 43 men and six women found Sunday were dumped at the entrance to the town of San Juan in the municipality of Cadereyta about 105 miles (175 kilometers) southwest of McAllen, Texas.

Graffiti around the town of 4,000 people mark it as Zetas territory, including "100% Zeta" painted on a stone arch welcoming visitors where the bodies were dumped and "Z’s" painted on the home of San Juan’s priest.

There have been 74 killings in the first four months of this year in Cadereyta municipality, compared to 27 over the same period in 2011 and seven in 2010, according to figures from Nuevo Leon state prosecutors.

The massacre follows the discovery of 14 men left in a van in downtown Nuevo Laredo on April 17 and 23 people found hanged or decapitated in the same border city May 4.

Eighteen dismembered bodied were left near Mexico’s second-largest city, Guadalajara, last week. Among the nine people identified in that attack were bricklayers, waiters and at least one student. None had criminal records.

Accused Zetas member Juan Carlos Antonio Mercado was arrested near Guadalajara last week in the kidnapping of 12 people. He told reporters that he and accomplices had been kidnapping people since mid-April at random and held them with the intent of dumping their bodies in the city center on May 10, Mexican Mother’s Day, but the police presence kept them from doing so.

Prosecutors in Jalisco state, where Guadalajara is, said the kidnapping plot fell apart when some victims escaped. The plot appeared to be linked to the discovery of the 18 dismembered bodies.

Drug violence has killed more than 47,500 people since President Felipe Calderon launched a stepped-up offensive when he took office in December 2006. The campaign has seen the two cartels emerge as Mexico’s two most powerful. At least one of the two cartels is present in nearly all of Mexico’s 32 states.

Their war started in earnest last fall with the dumping of 35 bodies in Veracruz, a strategic smuggling state with a giant Gulf port formerly controlled by the Zetas and recently taken over by a gang loyal to Sinaloa.

Guadalajara has long been controlled by gangs loyal to Sinaloa. Nuevo Laredo and Monterrey are considered territory of the Zetas gang, which was founded by deserters from the Mexican army’s special forces as the enforcement arm of the Gulf Cartel that historically dominated northeastern Mexico and the border along Texas. The groups split in early 2010, causing a bloody battle for territory that the Zetas have been winning. The weakened Gulf Cartel has started to align itself with Sinaloa to fight back.

Poire wouldn’t say which side was responsible for Sunday’s killings, though Domene said Sunday that it was the Zetas.

A state police investigator at the morgue, who spoke on condition of anonymity because he was not authorized to discuss the case, said some of the bodies were badly decomposed and some had their whole arms or lower legs missing.

San Juan, part of the Cadereyta municipality, is a town of farmers and factory workers near a refinery for Pemex, Mexico’s state-run petroleum company.

One resident who didn’t want to be identified for fear of retaliation said that the municipality has been without a local police for six years and that the Zetas have controlled San Juan for at least two years.

He said he believed that the Zetas’ enemies, who he wouldn’t name, dumped the bodies as a way to provoke authorities into cracking down on the Zetas. By Monday afternoon, both state police and Mexican soldiers were patrolling the town.

It’s a common tactic, known as "heating up the plaza," for drawing law enforcement to disrupt the activities of a cartel’s rival in its home territory, said Alejandro Hope, a security analyst and former official in Mexico’s CISEN intelligence agency.

"It puts the authorities in a reactive mode," Hope said.

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Associated Press writers Michael Weissenstein in Mexico City and Porfirio Ibarra Ramirez in Cadereyta contributed to this report.