In the western state of Michoacan, the burned bodies of 10 people were found in a charred pickup truck on Saturday.
The authorities initially believed the vehicle had exploded during a fuel theft. But Michoacan Governor Silvano Aureoles had said the town's police chief, his deputy and an investigator were detained for questioning.
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.
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," according to Godoy. A court will determine their legal status.