MEXICO CITY – Mexican President Felipe Calderon on Friday accused the gunmen who killed 72 illegal migrants in northern Mexico this week of “incalculable savagery” as his government attempted to depict the major drug gang implicated in the slaughter as weakened and desperate.

The discovery of the grisly massacre Tuesday night at a ranch near San Fernando, about 45 miles southwest of Brownsville, Texas, put the spotlight on Los Zetas, a crime syndicate based along Mexico’s Gulf Coast that has international tentacles.

The massacre  and its murky motives continued to shake Mexico on Friday.

A small car bomb blew up near the local offices of Mexican network Televisa farther south in Tamaulipas state, in Ciudad Victoria, causing damage but no injuries. Calderon himself also confirmed that two state criminal investigators who’d been assigned to probe the slayings had vanished.

In an interview on W Radio, Calderon searched for words to describe the contempt he felt for the gunmen who lined up the migrants mostly from Central America but also from Ecuador and Brazil and sprayed them with bullets.

“They are simply beasts,” Calderon said.

The sole apparent survivor, an Ecuadorean, feigned death and escaped the San Fernando ranch, stumbling with a bullet wound in his neck to alert Mexican marines who were stationed nearby.

 A top aide to Calderon blamed Los Zetas and said military pressure on the group had left it weakened and in need of reinforcements. Alejandro Poire of Mexico’s National Security Council said the gunmen had captured the migrants and given them a choice: Work for Los Zetas as gunslingers and peons, or face death.

When the 58 male and 14 female migrants resisted, they were killed.

“Rather than a kidnapping with an apparent financial aim, it was done fundamentally with the goal of detaining these people and forcing them to join the structures of organized crime,” Poire said, according to a transcript of his remarks, made in another radio interview, that Calderon’s office issued Friday.

Human and civil rights groups voiced outrage that the Calderon government used the massacre to defend its military campaign against drug cartels, even as the human toll grows.

Poire “tries to diminish the magnitude of the massacre, affirming that it is a sign that organized crime has been hit by the government,” 39 groups from around Latin America said in a statement.

However, an Austin, Texas-based strategic intelligence research group, Stratfor, said in a report Friday that Los Zetas, sometimes called simply the “Z’s,” indeed are hurting.

Los Zetas, which arose more than a decade ago as a paramilitary shock force for the Gulf cartel, broke away from the struggling Gulf drug lords in February and have been locked in a bloody war with other trafficking groups since then.

The group has branched into other areas of criminal activity, including piracy of consumer goods, extortion and taking control of human smuggling routes from traffickers known as coyotes, reaping from $2,000 to $10,000 per migrant.

Needing more gunmen against its rivals, Los Zetas called up Central American gang members with whom they’re allied, the Stratfor report said.

“This latest incident shows a continued desperation for manpower and ability to put boots on the ground to defend Los Zetas’ home territory,” it said.

 Another analyst of Mexico’s security situation, Edgardo Buscaglia, cast doubt on official accounts of the massacre and criticized Calderon’s handling of it.

“First, we have to find out whether they really were Zetas,” Buscaglia said, or whether they might have been crime gangs further down the chain.

Buscaglia said Calderon had glossed over evidence that all of Mexico’s major crime groups had grown stronger during his term, and that he has used military force against the cartels without taking other steps to deal with pervasive government corruption, which allowed the cartels to strengthen.

“The president, unfortunately, looks at all of this as a media war,” Buscaglia said. The portrayal of Los Zetas as weakened “is a marketing story” to deal with the international black eye Mexico has suffered with the massacre, he said.

“Every serious expert in the world agrees that Mexican organized crime groups are more powerful than ever before,” Buscaglia said.

Los Zetas have moved into parts of Central America. In March, police in Guatemala said the group had threatened to kill President Alvaro Colom, and a month later El Salvador’s president warned of Los Zetas’ presence in his country.

Police in Bolivia and Colombia also have reported ties between local drug clans and the violent Los Zetas.

• Borzou Darahi