Gunmen opened fire with assault rifles at an alcohol and drug rehabilitation center in Torreon, a city in the northern Mexican state of Coahuila, killing 11 people and wounding several others, state prosecutors said.

The attack happened on Tuesday afternoon at the La Victoria rehabilitation center in downtown Torreon, the La Laguna bureau of the Coahuila state Attorney General's Office said.

The gunmen arrived in at least two vehicles and opened fire on the patients who were at the center, the AG's office said.

Authorities have not determined exactly how many people were wounded in the attack.

Gunmen working for drug cartels have staged a number of attacks in recent years on drug treatment centers in the neighboring state of Chihuahua.

Gunmen attacked the Anexo de Liberacion y Adicciones treatment center last December in Ciudad Juarez, a border city in Chihuahua that is considered Mexico's murder capital, killing three people and wounding seven others.

At least 10 armed men went into the clinic and opened fire, officials said.

Gunmen killed 18 people at a drug treatment center located two blocks from a police station and the Santa Fe international bridge in Ciudad Juarez, which lies just across the Rio Grande from El Paso, Texas, on Sept. 3, 2009.

The attacks on rehab centers apparently target individuals who are using the facilities to sell drugs on rivals' turf, officials say.

Drug treatment centers in other parts of Mexico have also been targeted in recent years.

Gunmen killed 13 people on Oct. 24 at a drug rehabilitation center in the northwestern border city of Tijuana.

The attack occurred at the El Camino treatment center in east Tijuana, which is in Baja California state.