At least 24 people have been killed and seven others wounded after gunmen burst into an unregistered drug rehabilitation centre in central Mexico and opened fire, authorities said.
Police in the north-central state of Guanajuato said the attack occurred in the city of Irapuato, with three of the seven wounded in serious condition.
It appears the attackers shot every person inside the rehab centre, with state police saying nobody was abducted in the attack.
Guanajuato is the scene of a bloody turf battle between the Jalisco cartel and a local gang, and the state has become the most violent in Mexico.
No motive was given in the attack, but governor Diego Sinhue said drug gangs appeared to have been involved.
Mr Sinhue said: "I deeply regret and condemn the events in Irapuato this afternoon. The violence generated by organised crime not only takes the lives of the young, but it takes the peace from families in Guanajuato."
Mexican drug gangs have killed suspected street-level dealers from rival gangs sheltering at such facilities in the past.
But the latest assault was one of the deadliest attacks on a rehab centre since 19 people were killed in 2010 in Chihuahua city in northern Mexico.
Mexico has long had problems with rehab centres because most are privately run, underfunded and often commit abuses against recovering addicts.
The government spends relatively little money on rehabilitation, often making the unregistered centres the only option available for poor families.
And addicts and dealers who face attacks from rivals on the streets sometimes take refuge at the rehab clinics, making the centres themselves targets for attack.