A wounded Ecuadorean who escaped the killing ground in Mexico's Tamaulipas state told authorities that the migrants' abductors identified themselves as Zetas, a drug gang whose control of parts of the state is so brutal and complete that even many Mexicans avoid traveling its highways.
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.
Teresa Delagadillo, who works at the Casa San Juan Diego shelter in Matamoros just across from Brownsville, Texas, said she often hears stories about criminal gangs kidnapping and beating migrants to demand money - but never a horror story on the scale of this week's massacre.
"There hadn't been reports that they had killed them," she said.
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 called their journey "one of the most dangerous in the world" and said every year an untold number of migrants disappear without a trace.
Mexico's government has confirmed at least seven cases of cartels kidnapping groups of migrants so far this year, said Antonio Diaz, an official with the National Migration Institute, a think tank that studies immigration.
But other groups say migrant kidnappings are much more rampant. In its most recent study, the National Human Rights Commission said some 1,600 migrants are kidnapped in Mexico each month. It based its figures on the number of reports it received between September 2008 and February 2009.
Authorities said they were trying to determine whether the 72 victims in Tamaulipas were killed at the same time - and why. But government security spokesman Alejandro Poire noted migrants are frequently kidnapped by cartel gunmen demanding money.
Poire also said the government believes cartels are increasingly trying to recruit migrants as foot soldiers - a concern that has also been expressed by U.S. politicians demanding more security at the border.
If confirmed as a cartel kidnapping, the Tamaulipas massace would perhaps be the most extreme case seen so far and the bloodiest massace of Mexico's drug war.
"It's absolutely terrible and it demands the condemnation of all of our society," said Poire.
On Tuesday, Ecuadorean migrant Luis Freddy Lala Pomavilla staggered to the checkpoint with a bullet wound in his neck. He told the Mexican marines he had just escaped from gunmen at a ranch in San Fernando, 100 miles south of Brownsville, Texas.
He described a hellish scene of a room strewn with the bodies of 72 migrants, some piled on top of each other.
Photos by local media showed piles of people, some of them blindfolded and with their hands tied behind their back, slumped on top of each other along the cinderblock walls of an abandoned warehouse.
The 58 men and 14 women were killed by the Zetas gang, the migrant told investigators Wednesday. The gang, started by former Mexican army special forces soldiers, is known to extort money from migrants who pass through its territory.
The marines scrambled helicopters to raid the ranch, drawing gunfire from cartel gunmen. One marine and three gunmen died in a gunbattle. Then the marines discovered the bodies, some slumped in the chairs where they had been shot, one federal official said.
The migrant told authorities that his captors identified themselves as Zetas, and that the migrants were from Brazil, Ecuador, El Salvador and Honduras.
The Ecuadorean Embassy in Mexico said it was in contact with the survivor and was trying to determine if any of its citizens were among the dead.
Marcio Araujo, Brazil's consul general in Mexico, said documents found at the scene indicated at least four of the dead were Brazilian. Consular officials for El Salvador said they had no immediate information on whether any Salvadorans were among the victims.
The marines seized 21 assault rifles, shotguns and rifles, and detained a minor, apparently part of the gang.
Violence along the northeastern border with the U.S. has soared this year since the Zetas broke with their former employer, the Gulf cartel. Authorities say the Gulf cartel has joined forces with its once-bitter enemies, the Sinaloa and La Familia gangs, to destroy the Zetas, who have grown so powerful they now have reach into Central America.
It was the third time this year that Mexican authorities have discovered large masses of corpses. In the other two cases, investigators believe the bodies were dumped at the sites over a long time.
In May, authorities discovered 55 bodies in an abandoned mine near Taxco, a colonial-era city south of Mexico City that is popular with tourists.
In July, investigators found 51 corpses in two days of digging in a field near a trash dump outside the northern metropolis of Monterrey. Many of those found were believed to have been rival traffickers. But cartels often dispose of the bodies of kidnap victims in such dumping grounds.
Authorities are still digging in a mine shaft where seven bodies were removed over the weekend in the central state of Hidalgo. Two more bodies have been pulled out since, officials said Wednesday.
The Rev. Alejandro Solalinde, who runs a shelter in the southern state of Oaxaca, where many migrants pass on their way to Tamaulipas, said the Zetas have put informants inside shelters to find out which migrants have relatives in the U.S. - the most lucrative targets for kidnap-extortion schemes.
He said he constantly hears horror stories, including people who "say their companions have been killed with baseball bats in front of the others."
Solalinde said he has been threatened by Zetas demanding access to his shelters.
He said the gangsters told him: "If we kill you, they'll close the shelter and we'll have to look all over for the migrants."