Torreon - Eleven people were killed and at least nine others wounded in northwestern Mexico when a group of contract gunmen attacked a drug rehabilitation centre in the city of Torreon, police said.
"The armed attackers came on two pickup trucks, entered the centre and opened fire," a senior police official said on condition of anonymity.
The assault on the church-operated rehabilitation centre known as "Your Life on the Rock" occurred on Sunday night, police officials said.
The gunmen used AR-15 assault rifles and 9mm calibre pistols, the police noted, saying more people could have been wounded in the attack.
According to eyewitnesses, some of the victims walked away from the centre and disappeared before the police arrived.
The dead have not been identified yet.
The assault took place nearly a year after an attack on a similar rehabilitation centre in Torreon, located in Coahuila state, that also left 11 people dead and two wounded.
Five drug rehabilitation centres - all in the north of the country - were attacked in 2010.
Mexico has been struggling with a massive crime wave in recent years, with more than 50 000 people believed to have been killed in drug-related violence since 2006.
That year the government launched a military crackdown on the country's powerful drug cartels, which are themselves locked in brutal turf wars marked by macabre displays of violence such as beheadings and mass graves.
The government's official count of more than 47 000 dead has not been updated since last September.