Vigilantes – who were initially sanctioned and partially equipped by the Guerrero state governor – this week released 39 prisoners, after holding many of them for more than 30 days in a makeshift jail.

Under pressure from state authorities, the masked group handed 20 captives into state custody at a public ceremony early on Tuesday, and set the others free.

Meanwhile, the vigilante movement, which erupted across rural Guerrero last month, continues to spread and grow stronger. Up to 40 towns across the state are now running some form of community-deputised militia.

The militiamen — armed with machetes, rifles and shotguns — can be lethal: on Wednesday, Guerrero vigilantes shot and killed a man when a truckload of armed men ignored a vigilante checkpoint, sparking a gunfight in the town of El Refugio. Last month, a man was shot to death when he tried to escape vigilante detention in the town of Tixla.

The uprising began on January 5, in response to the kidnapping by an armed gang of a local commissioner, Eusebio Garcia, from the village of Rancho Nuevo. It was the latest in a string of kidnappings of local officials. In previous instances, ransom had been paid and the captives eventually released. This time, however, a group of outraged locals put on masks, grabbed weapons and headed out in search of the culprits.

Such vigilantism has since begun to spread beyond Guerrero; citizen militia groups are now reportedly active in the states of Michoacán, Oaxaca and Morelos, and, according to media reports, in the area surrounding Mexico City.

Filmmaker Ross McDonnell has been meeting with these villagers for the past month. Watch his video and return for more on this developing story.

See more coverage on SBS Dateline’s website.