The 72 people found murdered on a ranch in northeast Mexico are now believed to have been migrants bound for the US.
Mexican marines discovered the bodies of 58 men and 14 women after a clash with a suspected drug cartel in Tamaulipas state, which borders the US state of Texas.
One marine and three gunmen were killed.
An injured Ecuadoran man claiming to be the sole survivor of the massacre has been placed under federal protection. He alerted the military to the killings.
The man told police that the group had been kidnapped and killed by members of an armed gang heavily involved in the drug trade and organised crime and known for extorting migrants.
A Mexican security official Alejandro Poire said that a 'preliminary unconfirmed reports suggest (the victims) could have been immigrants from El Salvador, Honduras, Ecuador and Brazil.'
Director of Amnesty International Mexico Alberto Herrera said that if the victims turn out to be undocumented migrants 'it will turn into an emblem of the capacity or incapacity of Mexican officials to face up' to migrant abuses.
Mexico's Human Rights Commission has said that 10,000 undocumented migrants were abducted in Mexico over six months from September 2008 to February 2009 last year.
