Mexico City - Fed up with the police's failure to curb crime, armed vigilante groups have spread to at least four Mexican states, manning checkpoints, patrolling streets and in one case killing a "suspect" in a shootout.
The proliferation of self-policing poses a challenge to the government of President Enrique Pena Nieto, who inherited a drug war that had killed about 70 000 people in six years when he took office in December.
Raul Plascencia Villanueva, head of the National Human Rights Commission, said the vigilante trend must be rejected, warning that there is "a very fine line between self-defence organisations and paramilitary groups".
Although self-defence groups have long existed in Mexico, they began to expand last month, when hundreds of men donned masks and took up machetes and hunting rifles in the rural mountains of the southwestern Guerrero state.
In recent weeks, more groups have emerged in the neighbouring state of Michoacan, the southern state of Oaxaca and the central state of Mexico, which surrounds Mexico City.
In Guerrero, the civilians decided to police their own streets after a community leader was kidnapped in early January.
Detainees handed over to police
The vigilantes say the local police are too corrupt, unwilling or unable to stop drug gangs - a charge often levelled against police in Mexico.
The vigilantes detained 53 people, accusing them of a slew of crimes from murder to kidnapping and extortion, and held them in makeshift jails in remote villages.
They handed the last of their detainees to authorities this week after long negotiations, but the self-policing took a deadly turn on Wednesday.
Crisoforo Garcia, a leader of the vigilante movement, said a squad was on a "routine patrol" in the mountain village of El Refugio when it was attacked by a group of armed men, one of whom was killed in the resulting gunfight.
Interior Minister Miguel Angel Osorio Chong met with leaders of the Guerrero vigilante movement, agreeing to find a way to make the community police forces legal in return for the handover of 31 suspects.
Gerardo Rodriguez, a security expert and director of the Mexico Seguridad consultancy, said it was a "dangerous" deal, because giving legitimacy to these groups "shows the Mexican state's inability at the federal and local level to bring security and justice to these towns".
"It is a new flashpoint of insecurity for the Mexican state that can escalate," Rodriguez said.
Community police forces that are tolerated by the authorities have existed in Guerrero since 1995, allowing indigenous groups to mete out justice according to their own traditions.
But the vigilantes who have taken up arms in recent weeks were outside this local system, in which prisoners face public justice and sentences that include years of hard labour in various towns.
The growth of vigilante squads has sparked a debate among politicians, with some pointing fingers at state governors and other saying federal policymakers should do more to help.
"It is a governability crisis, because it demonstrates the total absence of the police," said Senator Ernesto Cordero, a leading member of the opposition National Action Party.
Manlio Fabio Beltrones, the leader of the ruling Institutional Revolutionary Party (PRI) in the lower house of Congress, said governors should "reconsider the way their governments work" if they "are not capable enough to offer protection, safety and justice" to their populations.
Guerrero's leftist Governor Angel Aguirre said federal lawmakers should provide more resources to state governments to improve security.
Fausto Vallejo, the PRI governor of Michoacan, suggested that the community police forces should be legalised and given training and equipment.
Pena Nieto has vowed to shift the focus of the drug war towards reducing the daily violence plaguing much of the country.
He wants to form a paramilitary "gendarmerie" to replace the thousands of troops deployed by his predecessor since 2006.
But the government has said soldiers will remain deployed until the level of violence falls.