Mexican troops have found 72 bodies at a remote ranch near the US border, the biggest single discovery since the launch of a drug war four years ago.
The bodies were discovered on Tuesday outside a town near San Fernando in the northern Tamaulipas state following a shootout with suspected drug-cartel member that left one marine and three suspects dead.
"The bodies were dumped about the ranch and were not buried. We are still investigating how long they had been there," a spokesman for the marine corps said on Wednesday.
Al Jazeera's Mariana Sanchez said officials confirmed that the bodies belonged to migrants from El Salvador, Honduras, Ecuador and Brazil, who were kidnapped by a drug cartel.
Formal investigations have not yet determined who was behind the massacre, but the federal official noted that the area is controlled by the Zetas drug cartel, which has expanded into kidnapping migrants.
Speaking on Wednesday, Felipe Calderon, the Mexican president, said: "Yesterday's crime, for example, shows [cartels'] beastliness, their brutality and their absolute lack of human scruples.
"I am sure we will still see a phase of very intense violence, principally among cartels."
The discovery of the bodies of 58 men and 14 women came when marines manning a checkpoint on a highway in Tamaulipas were approached by a wounded man who said he had been attacked by armed cartel members at a nearby ranch.
The man was placed under the protection of federal authorities. Navy aircraft were dispatched to the scene; when the attackers saw the planes, they opened fire on the marines and tried to flee in a convoy of vehicles.
Exchange of fire
After the exchange of fire, marines seized from the ranch assault rifles, bullets, uniforms and vehicles, including one with forged army licence plates.
Mexican drug cartels often use vacant lots, ranches or mine shafts to dump the bodies of executed rivals or kidnap victims.
The Zetas were members of Mexico's elite special forces trained to fight drug cartels, but they switched sides in the 1990s and became one of the country's most feared gangs, led by Heriberto Lazcano, known as The Executioner.
In May, authorities discovered 55 bodies in an abandoned mine near Taxco, a colonial-era city south of Mexico City that is popular with international tourists.
More than 28,000 people have died in drug-related violence in Mexico in recent years.