Armed vigilantes in southern Mexico killed a man in a shoot-out with a group of men they described as criminals.
It was thought to be the first death related to the month-and-a-half-old "self-defence" movement.
The confrontation near the town of Ayutla raised the stakes in a growing movement that has seen residents of several towns arm themselves with a motley assortment of old hunting rifles, shotguns and pistols while conducting patrols and manning checkpoints to fight crime spawned by drug cartels.
Bruno Placido, leader of the vigilante movement in the southern state of Guerrero, said one of the civilian patrols caught sight of a group of armed men, who opened fire on the patrol. "There was one killed on the side of the criminals," he said.
The masked vigilantes frequently stop passing motorists to search for weapons or people whose names are on hand-written lists of "suspects" wanted for crimes like theft and extortion.
The vigilantes have opened fire before on motorists who refused to stop, slightly wounding a pair of tourists from Mexico City visiting a local beach in early February.
The shoot-out on Wednesday came a day after the vigilantes freed the last of 42 people detained on suspicion of crimes ranging from theft to extortion and murder, a move that authorities had hoped spelled the beginning of a new, more regulated phase for the "self-defence" groups.
The Guerrero state government said the vigilantes turned 20 of the final detainees over to police. It said the other 22 had been suspected of lesser offences and were released on Tuesday because the vigilantes considered they had been sufficiently punished. "The state government foresees that the release of these detainees closes a chapter, and sets things on the road to institutionalising and regulating community police forces," it said.
Mr Placido, the vigilantes' leader, confirmed that some prisoners had been turned over.
State officials hope the vigilantes can be persuaded to join already-established "community police" forces that operate in some Guerrero towns.