Mexican marines guard mortuary workers outside a San Fernando, Mexico, funeral home where the bodies of 72 people allegedly killed by the Zetas drug gang are being kept. (Guillermo Arias/Associated Press)

Heavily guarded mortuary workers have begun to identify 72 migrants massacred on a ranch in northern Mexico near the U.S. border.

Meanwhile, human rights advocates demanded Mexico do more to stop the exploitation and abuse of migrants they say led to the slaughter.

Marines are protecting the pink, one-storey funeral home where the bodies were taken after being discovered Tuesday on a ranch. The victims were found bound, blindfolded and slumped against a wall. The building is in San Fernando, Mexico, about 160 kilometres south of Brownsville, Texas.

Jesus de la Garza, the assistant attorney general for Tamaulipas state, said Thursday that 15 bodies had been identified: eight from Honduras, four from El Salvador, two from Guatemala and one from Brazil. Diplomats from several of those countries travelled to Mexico to help identify the bodies, and Mexico's National Human Rights Commission sent investigators to monitor the process.

The government's chief security spokesman said it appears the migrants were slain because they refused to help a gang smuggle drugs.

"The information we have at this moment is that it was an attempt at forced recruitment," Alejandro Poire told W radio.

"It wasn't a kidnapping with the intent to get money, but the intention was to hold these people, force them to participate in organized crime — with the terrible outcome that we know."

The victims of what could be Mexico's biggest drug-gang massacre were traversing some of the nation's most dangerous territory, trying to reach Texas.

The lone survivor said the assassins identified themselves as Zetas, a drug gang that dominates parts of the northern state of Tamaulipas.

Meanwhile, two cars exploded early Friday morning in the northern Mexican state of Tamaulipas where officials were investigating the killing of the migrants.

No one was injured in the violence, but the front windows of a television station were blown out.