The arrests come amid a broadening government crackdown on the vigilantes, who took up arms a year ago to fight the Knights Templar drug cartel. The groups became popular in many towns because they were able to kick out the cartel, whose gunmen had demanded extortion payments from local residents, farmers and businesses.
But the vigilantes have brought their own form of lawlessness to largely agricultural Michoacan state, with rivalries, alleged thefts and possible links to a rival drug gang based in the neighboring state of Jalisco.
The federal government envoy to Michoacan, Alfredo Castillo, said at a news conference Monday that the leader of the vigilante group in the town of Yurecuaro, Enrique Hernandez Salcedo, had been arrested for illegal weapons possession. Castillo said prosecutors were also planning to charge him with ordering the killing of the mayor of the nearby town of Tanhuato, Gustavo Garibay.
Castillo alleged Hernandez Salcedo organized the March 22 killing of the mayor because Garibay "was opposed to the presence of the self-defense forces in the town of Tanhuato."
He said authorities had previously detained 14 members of the Yurecuaro vigilante group under Hernandez Salcedo's command. Five of those, including one who was later found dead, allegedly participated in the killing of Garibay, waiting for the mayor outside his house and attacking him as he emerged.
Some of them implicated their leader in the crime, Castillo said.
The town of Tanhuato was apparently strategic because it lies near Michoacan's border with Jalisco. Castillo said gangs would often kill people in Michoacan and take their bodies to Jalisco for burial in clandestine graves, though he did not say whether the vigilantes had been involved in those killings.
Garibay was wounded in a similar attack in 2012, and his secretary was slain in February. The mayor was a member of the National Action Party of former President Felipe Calderon, a Michoacan native who began his military offensive against drug gangs in the state.
The government had tolerated thousands of vigilantes toting assault rifles, until the March 11 arrest of vigilante leader Hipolito Mora on charges of having participated in the killings of two members of a rival vigilante faction.
In the last week, Castillo said, federal and state authorities have arrested more than 50 members of the self-defense forces on various charges. There have been reports that some vigilantes are taking advantage of their position to loot properties abandoned by Knights Templar bosses.
Estanislao Beltran, the spokesman for the vigilante movement, said he knew Hernandez Salcedo as a fellow self-defense leader, but said the movement would not cover up for anyone accused of crimes.
"Anybody who is guilty of something has to pay for it," Beltran said.