Prosecutors in Mexico say gunmen have killed at least 15 people at a car wash in the west of the country.
At least two others were injured in the attack early on Wednesday in Tepic, Nayarit state.
Local media reported that the victims were mostly young men who lived at a drug rehabilitation centre.
Police say rehab facilities are increasingly being targeted by drug cartels who suspect the clinics of harbouring members from rival gangs.
The attack came only days after gunmen shot more than a dozen recovering addicts in Tijuana, in Baja California.
And in September, 28 patients were killed in attacks on two rehab clinics in Ciudad Juarez.
The victims in that attack were lined up against a wall and murdered in what police said were reprisal killings.
Five men belonging to the powerful Sinaloa drug cartel have been arrested for the Ciudad Juarez murders.
Some 28,000 people have died in drugs-related violence in Mexico since 2006.
More than 7,000 people have died in the violence in 2010 - making it the bloodiest year since President Felipe Calderon dispatched some 50,000 troops to take on the drug cartels in 2006.
Mr Calderon, who was attending a forum on security as news of the latest massacre broke, called a minute of silence for victims of the bloodshed, the Associated Press reported.
The latest violence came shortly after Mr Calderon told the BBC the US should do more to reduce the demand for drugs that is fuelling violence in Mexico.
He told the Hardtalk programme that more should also be done to stem the flow of illegal weapons from the US.
President Calderon and other regional leaders have urged Californian voters to reject moves to legalise marijuana in their state.
Last week, police in Tijuana destroyed 134 tonnes of cannabis - the largest drugs haul ever seized in the country.