MEXICO CITY -- Mexican marines have arrested seven gunmen suspected of killing 72 Central and South American migrants in the worst drug cartel massacre to date, the government announced Wednesday.
Four of the suspects were arrested after a Sept. 3 gunbattle with marines, and the other three were captured days later, spokesman Alejandro Poire said at a news conference.
Poire alleged the seven belong to the Zetas drug gang, but he gave no further details on their identities or what led to their arrests.
Investigators believe the migrants were kidnapped by the Zetas and killed after refusing to work for the cartel.
The arrests "will help determine exactly what happened in San Fernando, Tamaulipas, and it's a significant step toward ending the impunity surrounding assaults on migrants by organized crime," Poire said.
Marines -- alerted by an Ecuadorean survivor who managed to escape the mass killing -- found the bodies of the migrants Aug. 24 at a ranch in Tamaulipas, a state that borders south Texas. The bodies, bound and blindfolded, were found lying in a row beside a wall, some slumped on top of each other.
A total of eight suspects are now in custody: Marines arrested a teenager after a shootout with gunmen at the ranch the day they discovered the bodies. Three gunmen were killed during that battle.
In addition, marines last week found the bodies of three other men suspected of participating in the massacre after an anonymous caller told authorities where to find them. Officials say they have no information on who made the call, but in the past drug gangs have handed over suspects in especially brutal killings that draw too much attention.
A Honduran man who also survived the slaughter and is under police protection in Mexico later identified the three dead men as having been among the killers.
The Ecuadorean survivor, who has since returned to his country, told Mexican investigators the killers identified themselves as Zetas and killed the migrants when they refused to join the gang.
Salvadoran President Mauricio Funes said a Salvadoran citizen also survived the massacre and had made it to the United States. Poire said Wednesday that Mexican authorities have no information to confirm that person was among the migrants captured by the Zetas.
The latest arrests were announced one day after authorities found the bodies of two men believed to be those of a state detective and a local police chief who participated in the initial investigation of the massacre.
The two officers went missing a day after the migrants were found in San Fernando, a town about 100 miles (80 kilometers) south of Brownsville, Texas.
The Tamaulipas state Attorney General's Office said identification documents found on the bodies matched those of the missing officials, state detective Roberto Suarez Vazquez and Juan Carlos Suarez Sanchez, who was head of the Public Safety department of San Fernando.
The two bodies were found in a field about 30 miles northeast of San Fernando. Investigators are conducting DNA tests to confirm the identities, suggesting the condition of the bodies are too poor to permit visual identification.
Suarez Vazquez filed the initial crime report on the bodies of the massacre victims, and Suarez Sanchez accompanied him during that task.
Their slayings -- if confirmed -- would be another demonstration of how freely drug gangs operate in Tamaulipas, despite the presence of troops and federal police. It would show that far from fleeing, gang henchmen kidnapped two top police officers even after Mexican marines swarmed the area after the massacre.
Tamaulipas has become of one Mexico's bloodiest drug trafficking battlegrounds this year since the Zetas and the Gulf cartel ended their alliance. The Zetas began as a group of Gulf cartel henchmen but have grown into a drug trafficking gang in their own right, and have long been known to be involved in migrant smuggling.