Juan Pablo Diaz locked the door of the Casa Aliviane rehabilitation centre for the final time as the mattresses of the 18 who died here were loaded on to a truck.

“This was my life’s work,” he said, tears streaming down his face. The walls and floors have been scrubbed of blood and the only signs remaining of the massacre that took place here are the bullet holes that scar every surface.

Gunmen stormed the building, forcing patients and helpers into an inner courtyard before shooting dead all but three of them. The attack was the 11th on a rehabilitation centre in this, Mexico’s most violent city, since August last year, with ten people killed in a similar incident two weeks earlier. The motive for the attacks remains unclear.

Mr Diaz had left someone else in charge on the night last month when the killers struck while he took his children to a karate class. “When I came back there were bodies everywhere,” he told The Times. One of the dead was his nephew. The survivors said that six gunmen had burst in, ordering the patients out of their dormitories and into a narrow courtyard. “They even killed the dogs,” Mr Diaz said. “If I had been here they would have killed me too.”

The authorities claim that the killings are carried out by drug cartels because the centres are harbouring rival gang members, but this is denied vehemently by the victims. Mr Diaz suggests that the cartels may be trying to scare addicts away from rehabilitation centres or — in their battle with the thousands of soldiers who have poured into the city as part of President Calderón’s war on the drugs trade — are stepping up the levels of brutality simply to show the extent of their reach. “Probably their aim is to show their power, to show what they can do,” he said.

The authorities say that five members of the notorious Sinaloa cartel, arrested recently for 45 alleged executions, were responsible for the attack, and the Mayor of Ciudad Juárez, José Reyes Ferriz, told The Times that the crime had been solved. Mr Diaz is not convinced. “The police never do anything,” he said. “Who knows when it will stop?”

Carlos Chavira, president of the Public Security Council, also expressed doubt about the arrests. He said an investigation was needed to determine the real reasons for the attacks on the centres.

Times Recommends

• Please help: maimed children of war want their lives back
• Cabal seizes diamond riches to buy power
• White House gatecrashers got to the president