The victims were found in a charred pickup truck in the western state of Michoacan on Saturday. The authorities initially believed the vehicle had exploded during a fuel theft.
But the state's attorney general, Jose Martin Godoy, said the mayor of the municipality of Alvaro Obregon and four police officers, including a commander, were arrested over the deaths.
Michoacan Governor Silvano Aureoles had earlier said the town's police chief, his deputy and an investigator were detained for questioning.
The investigation is "strengthening the hypothesis" of involvement by "municipal officers," Aureoles told reporters, adding that there were also "some direct accusations" against the mayor.
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Police officers in three Alvaro Obregon municipal vehicles detained a group of people on Friday night at a grocery store in the town of Cuitzeo and forced them into a red pickup truck, Godoy said.
Witnesses said the mayor, Juan Carlos Arreygue, had ordered police to detain the group and that he was present when they were arrested, he said.
One of those detained by the officers had "differences of a personal nature" with the mayor, the prosecutor said.
"After the arrests, under instructions from the mayor, the civilians were secured, subdued and forced into a van," he added, citing unidentified witnesses.
"Then, they took the bodies to a field in the municipality of Cuitzeo where they set them on fire."
The bodies were found in a burned vehicle on a dirt road in Cuitzeo, an area where gangs are known to steal fuel from pipelines.
Prosecutors are continuing the investigation to "determine the level of participation of public servants in this terrible event," Godoy said. A court will determine their legal status.
Latest scandal
Mexican police and troops have faced a slew of allegations of human rights abuses since the country's battle against drug trafficking escalated in 2006 with the deployment of armed forces.
In a case that drew international condemnation, the authorities say local police abducted 43 students in the southern state of Guerrero in 2014 and handed them over to a drug cartel that killed them and incinerated their bodies.
The mayor of the city of Iguala, who remains in jail, was accused of ordering police to confront the students.
The deaths in Cuitzeo also put a new spotlight on Michoacan, a state that has been relatively peaceful recently after years of brutal drug violence.
Murders, kidnappings and extortions by the pseudo-religious Knights Templar drug cartel prompted farmers to form vigilante militias in 2013.
The cartel has been largely dismantled and the vigilante forces dissolved, the authorities say.
But hundreds of soldiers were recently deployed as reinforcements in a sign that the state remains a concern.
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