(CNN) -- Authorities were investigating Monday after at least 11 people were killed when gunmen attacked a drug rehabilitation center in northern Mexico, state media reported.
The attack occurred around 10 p.m. Sunday in the northern city of Torreon, the state-run Notimex news agency said, citing prosecutors. At least 15 people were injured.
Photos of the scene showed Red Cross ambulances outside the Your Life on the Rock rehabilitation center. Inside, a man was slumped on a sofa, a pool of blood beside him.
Drug treatment centers have been targets of attacks in northern Mexico, a region where violence has increased as warring cartels battle over territory and power and government troops patrol the streets.
Last year, prosecutors said 13 people were killed after gunmen opened fire in another rehabilitation center in Torreon.
In 2010, at least 30 armed men invaded a drug treatment center in the northern city of Chihuahua, killing 19 patients and wounding four.
At the time, witnesses told authorities the armed men marched 23 people outside, lined them up and shot them, the state-run Notimex news agency said, citing a local police official.
In 2009, an attack on a rehab facility in Ciudad Juarez left 17 dead and two wounded. Jose Reyes Ferriz, then the city's mayor, said at the time that authorities believed a rival drug gang shot the men.
The motive for Sunday's shooting was not immediately clear.
Several factors have made rehab centers targets, said Edgardo Buscaglia, an expert on organized crime and president of the Mexico-based nonprofit Instituto de Accion Ciudadana (Citizen Action Institute).
"The people who go there are usually involved in buying or selling drugs," he said. "We're talking about urban gangs or youth gangs that are in charge of delivering these drugs to the final consumers in the urban markets."
Some of them may have talked with police, drawing ire from former associates, he said. Others may be targets for rival gangs seeking revenge.
"They kill them because these kids are either witnesses for the authorities or ... some of those kids may be working in the selling of drugs. Adversary groups kill them in the same ways they kill politicians working for the adversary groups," he said. "Once you work for different organized crime groups, it's very hard to get away from it."
In 2010, Mexico's then-health secretary said rehab centers in the northern state of Chihuahua had been forced to close because of pressure from gangs trying to sell drugs there.
Last month, Mexico's interior minister said the Zetas drug cartel and members of the allied Gulf and Sinaloa cartels were in a fierce feud in the region. He blamed the battle between rival cartels for the 49 decapitated and dismembered bodies left along a highway in the northern city of Cadereyta, Nuevo Leon.
More than 47,500 people have been killed in drug-related violence in Mexico since President Felipe Calderon began a crackdown on cartels in December 2006, according to government statistics.
CNNMexico.com's Rodrigo Aguiar and CNN's Rafael Romo contributed to this report.