Officials announced the reward Monday for two suspects known by the nicknames "The Coyote" and "The Scorpion."  They also offered a reward of $411,000 for a third suspect in the mass killings. At least seven people already have been arrested in the case.

The bodies of the migrants were discovered on a ranch in the northeastern Mexican state of Tamaulipas, which borders the U.S. state of Texas. Mexico is a transit point for illegal migrants from Latin America and elsewhere seeking to reach the United States.  Many fall victim to gangs and organized crime.

Authorities have implicated the Zetas drug gang in the massacre of the 72 migrants.  

The Zetas began as a Mexican military unit that defected and began working with the Gulf cartel, based in Juarez, Mexico, across the Rio Grande river from the U.S. city of El Paso, Texas. The Zetas split from the Gulf cartel last year. The two groups are now fierce rivals.

More than 34,000 people have been killed in Mexico's drug war since President Felipe Calderon took office in late 2006 and began cracking down on the cartels. The cartels are locked in a violent battle for control of trafficking routes into the United States.