Mexican troops say they have discovered 72 bodies at a ranch in the north of the country.
The bodies were found after a shoot-out with suspected drug cartel gunmen in which three gunmen and one marine were killed, the military said.
Unconfirmed reports say the 58 men and 14 women, found in Tamaulipas state, were migrants trying to reach the US.
More than 28,000 people have died in drug-related violence in Mexico over the last four years.
The shoot-out, near San Fernando, happened after an injured man turned up at a navy checkpoint saying he had been attacked by cartel gunmen, Mexican media reported.
Troops raided the ranch, launching an aerial assault. They arrested one underage suspect, military said.
They found the bodies during a search of the site, also discovering a cache of weapons, ammunition and uniforms.
It appears to be among the biggest mass graves linked to drug-related violence to be uncovered in Mexico.
It was not immediately clear exactly where the bodies were found or for how long they had been there.
The man who escaped the shooting said he and the victims had been trying to reach the US illegally when they were kidnapped by a criminal gang, an unidentified federal official told Mexico's Universal newspaper.
The gang offered them work but opened fire when they refused. The unnamed man said he was from Ecuador while the victims were from various countries, including Brazil.
There has been no official comment on the media reports.
Tamaulipas is one of the states worst affected by the violence, and has been the scene of fierce fighting between the Zetas and Gulf cartels.
The navy issued a statement saying the government condemned the "barbarous acts committed by criminal organisations".
"Society as a whole should condemn these type of acts, which illustrate the absolute necessity to continue fighting crime with all rigour," the statement said.
In recent months an increasing number of mass graves have been discovered.
In June, police recovered 55 bodies from an abandoned mine near Taxco, in Guerrerro state.