24 people were killed and seven others left wounded, after gunmen burst into an unregistered drug rehabilitation centre in central Mexico and opened fire.
According to Police in the north-central state of Guanajuato, the attack occurred in the city of Irapuato.
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.
Three of the seven wounded are reportedly in serious condition, the official said.
The police said the attackers shot everyone at the rehab centre, adding that nobody was abducted. It is the second such attack to take place in Irapuato in the past month.
Governor Diego Sinhue said drug gangs appeared to have been involved in the latest attack.
Sinhue said: “I deeply regret and condemn the events in Irapuato this afternoon.”