Nine people, including at least four boys aged under 18, have been shot dead in an amusement arcade in the city of Uruapan in western Mexico.
Investigators say the arcade is used by the Los Viagra gang to sell drugs.
The gunmen are thought to belong to the rival Jalisco New Generation cartel.
The killings come just days after police detained a suspected local leader of Los Viagra, an arrest which triggered shootouts between the security forces and gang members.
After Luis Felipe Barragán was detained, his men set alight cars and trucks to block local roads and opened fire on police officers, wounding two.
Witnesses told investigators that four armed men entered the arcade and asked for two members of the Los Viagra gang known as “The Russian” and “The Bald One”.
Without waiting for an answer they then proceeded to shoot those inside the arcade at short range. Police found 65 bullet casings.
The attack on the arcade came a day after prosecutors in the city said they had found a mass grave containing 11 bodies, some of which showed signs of torture.
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Uruapan is located in Michoacán, one of the states worst affected by the rise in violence which Mexico has seen in recent years.
The BBC reports Los Viagra has its stronghold in the Tierra Caliente farming region in Michoacán state and was formed by the Sierra Santa brothers. The gang’s leader is the oldest of the eight siblings, Nicolás Sierra.
Like many gangs in Mexico, they have formed – and broken – ties with other criminal groups over time, leading to deadly turf wars.
They were reportedly once allies of the Jalisco New Generation cartel but fell out over who should control the smuggling of synthetic drugs such as methamphetamines and crystal meth.
The gangs are also involved in extortion, kidnappings, illegal logging and have been trying to gain control of the lucrative trade in avocados.
In response, local avocado growers have created their own vigilante force.
The rivalry between the Jalisco New Generation cartel and Los Viagra is thought to be behind many of the killings and gruesome displays of violence in the area such as when nine bodies where hung from a bridge in Uruapan.
