Mexican officials say the 72 people found murdered at a ranch close to the US border were migrant workers trying to reach the United States.
Soldiers came across the bodies of 58 men and 14 women after a shoot-out between suspected drug cartel members and security forces.
Its the biggest single discovery of its kind in Mexico’s increasingly bloody drug war.
The killings happened in Tamaulipas state, around 150 kilometres from the border with Texas.
Forces were alerted by a migrant who escaped the carnage.
The unnamed witness claimed the gang shot dead the people after they refused to work for the cartel.
Three gunmen and a marine died in the clashes, one gang member was arrested, while others made their getaway.
The victims are believed to come from Ecuador, El Salvador, Honduras and Brazil they were kidnapped by the gang as they headed to Texas.