SAN FERNANDO, Mexico - Working under heavy security in a region controlled by a brutal drug gang, authorities and diplomats began the gruesome task Thursday of identifying the bodies of 72 Central and South American migrants killed just 100 miles from their destination.

Refrigerated boxes were being readied to store the bodies from what could be Mexico's biggest drug-gang massacre. The bodies were found blindfolded and slumped against a wall at a remote ranch Tuesday near San Fernando, a town of about 30,000 on Mexico's east coast.

They had been trying to reach the Texas border, traversing some of Mexico's most dangerous territory.

Marines guarded the building holding the bodies, using vehicles to block the access several streets away. At least one refrigerated truck was parked nearby.

The Mexican government flew diplomats from Brazil, Ecuador and El Salvador - where the migrants were believed to be from - directly to San Fernando to identify the bodies.

The gruesome killing was discovered after the lone survivor, 18-year-old Luis Freddy Lala Pomavilla, staggered wounded to a marine checkpoint on a highway and told authorities about the slaughter.

His family told Ecuador television Thursday that Lala left his remote town in the Andes mountains two months ago in the hopes of reaching the U.S.

"I told him not to go but he went," one of his brothers, Luis Alfredo Lala told Ecuavisa television from Lala's home town.

Lala's parents already live in the United States and send money home to the family, and Lala had been the primary caretaker for his eight siblings and his grandmother, according to a cousin, Maria Ignacia Gualga.

Lala, who was recovering from a gunshot to the neck at a Mexican hospital, has a 17-year-old pregnant wife in Ecuador, Maria Angelica Lala. She told Teleamazonas that her husband had paid $15,000 for the smuggler who was supposed to guide him to the United States.

That smuggler apparently tried to hide Lala's fate from his family, calling Wednesday to tell her that Lala had safely reached Los Angeles. It was the day after Mexican marines acting on Lala's tip had raided the ranch and found the slain migrants, 14 of them women.

Drug gangs in Mexico often force human smugglers to abandon their migrants.

Lala said his captors had identified themselves as Zetas, a drug gang whose domination of parts of the northern state of Tamaulipas is so complete that even many Mexicans avoid traveling its highways.

If confirmed as a cartel kidnapping, it would be the most extreme case seen so far and the bloodiest massacre since President Felipe Calderon began a crackdown on drug gangs in late 2006. More than 28,000 people have died in drug-related violence since then.

Calderon condemned the massacre as the work of desperate cartels.

They "are resorting to extortion and kidnappings of migrants for their financing and also for recruitment because they are having a hard time obtaining resources and people," he said in a statement Wednesday night.

But advocates said Mexico's indifference to migrants' exploitation enables such heinous crimes to flourish.

"We disagree with the government that is a consequence of battles between criminal groups," said the Rev. Pedro Pantoja, director of the Casa del Migrante in Saltillo in neighboring Coahuila state. "The permissiveness and complicity of the Mexican state with criminals ... is just as much to blame."

In its most recent study, the National Human Rights Commission estimated nearly 20,000 migrants are kidnapped each year based on the number of reports it received between September 2008 and February 2009 - numbers the federal government has dismissed as unreliable.

In an April report, Amnesty International called the plight of tens of thousands of mainly Central American migrants crossing Mexico for the U.S. a major human rights crisis.

The report said that although the Mexican government has made some small improvements, it continues to give the issue low priority, despite the widespread involvement of corrupt police.

Kidnappings and attacks on government security patrols are rampant in the highways surrounding San Fernando, where armed men claiming to belong to the Zetas roam freely and the police station is pockmarked with bullet holes from March shooting.

Last month, the bodies of 15 people were dumped in the middle of the highway from San Fernando to Matamoros, a city across the border from Brownsville, Texas.

The region is at the end of a traditional migration route for Central and South Americans who travel up Mexico's Gulf coast toward southern Texas. Violence has soared there this year since the Zetas broke with their former employer, the Gulf cartel, sparking a viscous turf war.

Authorities said they were trying to determine whether the 72 victims in Tamaulipas were killed at the same time - and why.

Migrants running the gauntlet up Mexico to reach the United States have long faced extortion, violence and theft. But reports have grown of mass kidnappings of migrants, who are forced to give the telephone numbers of relatives in the United States or back home who are then required to transfer ransom payments to the abductors.

But migrants and immigration activist say they had never heard of an atrocity on the scale of the San Fernando massacre. Some have decided the journey has finally become too dangerous.

Almost 20 migrants staying at the Casa del Migrante shelter outside Mexico City turned back to their countries after hearing of the killings this week, said shelter worker Hector Lopez, a Nicaraguan who abandoned his own journey three months ago.

"I wanted to go reach the United States but when I saw what the situation was, what was happening to other migrants, I realized things could get worse for me," he said.

But others refused to turn back, even though they admitted they were stunned by news of the massacre.

"We run from the military, the authorities, the police and now the criminals, the Zetas. We are just poor people, we're just passing through. Why do they have to do this to us?" said Wilber Cuellar, a migrant from Belize who was staying at the shelter.

Cuellar, 35, who said he has been deported six times from the United States and once from Canada, where he had worked at a chicken packing plant, vowed the massacre would not deter him.

"I'm not afraid. I'm prepared to die," he said. "I'm tired of suffering in this world."