A gunfight between security forces and armed civilians in Mexico's southwestern state of Guerrero killed 15 people on Tuesday, authorities said, the second mass killing to shake the country in as many days.
Guerrero state public security spokesman Roberto Alvarez said 14 civilians and one soldier died in the shootout in the municipality of Tepochica, near Iguala, a city notorious for the 2014 disappearances of 43 student teachers.
A photograph of the aftermath seen by Reuters showed two slain civilians, one of them hanging limply off the side of a battered pick-up truck that had been riddled with bullets as security forces patrolled the area.
Defending his security strategy after suspected cartel gunmen killed 13 police a day earlier in the neighbouring western state of Michoacan, President Andres Manuel Lopez Obrador blamed past administrations for Mexico's chronic violence.
However, the latest carnage is likely to increase pressure on Lopez Obrador to get a grip on a problem he pledged to tackle when he took office last December.
Guerrero and Michoacan are two of Mexico's most violent and lawless states, where rival drug gangs have battled to control smuggling routes to the Pacific and interior of the country.
The leftist Lopez Obrador told a news conference the ambush in Michoacan was "very regrettable" but reiterated that his commitment to greater spending on security and fighting the root causes of violence would eventually pay dividends.
"I'm optimistic we'll secure peace ... we're completely dedicated to this issue, but (past governments) allowed it to grow," said Lopez Obrador, who has criticized past efforts for taking a confrontational approach to battling crime.
Homicides in Mexico this year are on track to surpass last year's record.
Photos of the Michoacan crime scene on social media showed bullet-riddled police vehicles set on fire, as well as bodies of dead officers on the ground.
After taking office, Lopez Obrador created a militarized National Guard police force to contain the violence.
But many of the National Guard have instead been deployed to police Mexico's borders to placate US President Donald Trump, who has threatened to impose tariffs if Lopez Obrador does not reduce the flow of US-bound migrants from Central America.
The city of Iguala pitched Mexico's chronic security problems into the glare of international media after the 43 trainee teachers were abducted by a drug gang in cahoots with corrupt local police on the night of Sept. 26, 2014.
The resulting scandal battered the reputation of Mexico's former president and helped propel Lopez Obrador into office.
The last government said the drug gang killed and incinerated the youths, although investigators only ever definitively identified the remains of one of them.