MEXICO CITY - A survivor of a massacre in northern Mexico told investigators that the 72 people found dead on a ranch were undocumented migrants who were kidnapped by a drug gang on their way to the U.S. border.

Mexican authorities discovered the bodies late Tuesday about 100 miles south of Brownsville, Tex., near the town of San Fernando.

The surrounding state of Tamaulipas is the scene of a vicious struggle between rival drug mafias and Mexican forces that has left hundreds dead and important trade cities besieged by gun battles, kidnapping and extortion.

The bodies of 58 men and 14 women were discovered after Mexican marines manning a checkpoint were approached by a wounded man who said he had been held hostage and shot by gang gunmen at a nearby ranch.

The survivor, who has been hospitalized and said he was originally from Ecuador, told his story to state prosecutors, according to a report Wednesday on the Web site of the newspaper Reforma.

The witness said about 75 migrants from Central and South America were traveling together toward the Texas border when they were kidnapped by armed men, who authorities suspect may work for the paramilitary drug cartel known as the Zetas.

The survivor said that when the migrants refused to pay money, their captors began shooting them. When Mexican forces arrived at the scene, the assailants scattered and fled. No arrests have been announced. It was unclear whether the dead had been shot all at once or over several days.

Illegal migrants traveling through Mexico on their way to the United States often face a harrowing journey - robbed, raped and sometimes killed by smugglers and crime gangs, which often work alongside corrupt police.

Mexico detained and deported more than 64,000 illegal migrants last year, according to the National Migration Institute. Many more make it across the U.S. border. The National Commission on Human Rights, a government agency, estimates that 20,000 are kidnapped in Mexico each year.

While they are held for ransom, migrants are routinely threatened with execution, beaten with clubs and shocked with electricity.

As Mexico's U.S.-backed, military-led war against criminal organizations rages on, leaving more than 28,000 dead, authorities are discovering mass graves.

In July, investigators found 51 corpses near a trash dump outside the northern city of Monterrey. Many of their identities remain unknown.

In May, authorities discovered as many as 64 bodies in a ventilation shaft for a shuttered mining operation near the tourist town of Taxco. Many of the bound and gagged victims were alive when they were thrown down the hole.