MEXICO CITY—Gunmen from a drug cartel appear to have massacred 72 migrants from Central and South America who were on their way to the U.S., a grisly event that marks the single biggest killing in Mexico's war on organized crime.

Mexican marines discovered the 72 bodies—58 men and 14 women —on Tuesday after the lone survivor of the massacre, a wounded migrant from Ecuador, stumbled into a Navy checkpoint the previous day and told of being shot on Monday at a nearby ranch, Mexican officials said on Wednesday.

When the marines went to investigate, they were met with a hail of gunfire from cartel gunmen holed up at the ranch, which sits 90 miles from the U.S. border. One marine and three alleged gunmen died during a two-hour battle, which ended when the gunmen fled in a fleet of SUVs, leaving behind a cache of weapons.

The Ecuadorean migrant told investigators that his captors identified themselves as members of the Zetas drug gang, said Vice Adm. Jose Luis Vergara, a spokesman for the Mexican navy.

"This illustrates that organized crime has no limits or moral qualms about what they are prepared to do," Alejandro Poire, head of the government's national-security council, told a news conference.