Thirteen people have been reported missing following the withdrawal of vigilantes from a violence-wracked city in southern Mexico, a human rights official said Tuesday.

The disappearances add to confusion surrounding the takeover of the city of Chilapa by so-called "community police" earlier this month.

The vigilantes, claiming to be self-defense forces from surrounding towns, took over Chilapa on May 9, supposedly to end drug violence in a city being fought over by two rival drug gangs, the Rojos and the Ardillos.

They disarmed the local police and forced the city police chief to resign because they thought some of the police worked for the Los Rojos gang. The vigilantes later returned the police weapons, a new police chief was named and state and federal police were sent in to take charge of security.

But some residents claimed the vigilantes worked for Los Ardillos.

Ramon Navarrete, the head of the Guerrero state human rights commission, said 13 people had been reported missing between May 9 and mid-month, when the vigilantes withdrew.

Navarrete said residents apparently waited for the vigilantes to withdraw before filing police reports.

"There appear to be a number that have gone unreported, because some people are afraid to file complaints ... That is something we can understand," Navarrete said.

Violence had gotten so bad in Chilapa that a mayoral candidate was killed there in early May, and a candidate for governor was stopped by armed men in April.

Ten bodies and 11 severed heads were discovered in clandestine graves in Chilapa in January. Most of the bodies had their hands tied and showed signs of torture.

In late November, 11 other headless bodies were found in the area. In 2014, a Ugandan missionary priest was kidnapped from the area and later found dead.

Residents say they think that a much higher number of Chilapa residents have gone missing

Chilapa resident Jose Diaz Navarro, whose own family has suffered disappearances, said "there are a lot of disappeared people, but very few complaints are filed."

In the five days that the vigilantes were in Chilapa, Diaz Navarro said at least 16 people, and possibly as many as 30, disappeared.

He also questioned who the vigilantes really are.

"We call them gang members, because they never really appeared like community police," Diaz Navarro said. "They did not show any document, they had high-powered rifles, so we cannot accept that they were community police."

Community police forces do exist in some Guerrero towns, but they are designated by community assemblies. They are limited by law to policing their own towns, usually with low-caliber, single-shot rifles or shotguns.

Chilapa Mayor Francisco Javier Garcia asked the federal government for more support.

"I think Chilapa truly has a very serious problem ... It could ignite the whole state," said Garcia. "We are asking the president ... to pay attention to our city."

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