It was a messy business, clearing-up after the carload of masked gunmen who burst into the function room at a suburban hotel in the city of Torreon in northern Mexico last Saturday night and began randomly spraying bullets into a crowded dancefloor full of young men and women.
A total of 17 people were killed and dozens more injured in the apparently indiscriminate attack, which lasted two minutes and, according to witnesses, was carried out by four suspects, who fired 120 bullets at revellers attending a birthday party organised via Facebook.
Yet when detectives began picking through the blood-stained evidence left behind after the latest atrocity in a drug-related crime wave which Mexico's government estimates has killed 25,000 people in the past five years, they made a discovery that turned an ordinarily grisly massacre into something astonishing.
Forensic examinations of the bullet casings found on the function suite's floor revealed that they had all been fired from one of four AR-15 assault rifles which were supposed to belong to guards at a prison in the neighbouring state of Durango.
After further investigation which saw the director of the jail in the town of Gomez Palacio and three of his officials placed under house arrest, the Attorney General announced that he had uncovered a conspiracy shocking even by the standards of a country that has grown used to shocking levels of violence and corruption.
For several months, according to detectives, guards at the prison have apparently been releasing select groups of inmates, lending them automatic weapons, and sending them off in official vehicles to carry out drug-related killings. After carrying out the attacks, the prisoners have been driving back to jail, handing over the firearms and returning quietly to their cells. The authorities that help facilitate their murderous sprees are said to have received huge cash payments in return.
No one knows exactly how many of the audacious murders and contract killings have been plotted behind bars, but Ricardo Najera, a spokesman for the Attorney General's office, said that ballistics tests had linked the AR-15s to two attacks at bars in Torreon in February and May, in which 16 people were killed. "According to witnesses, the inmates were allowed to leave with authorisation of the prison director to carry out instructions for revenge attacks using official vehicles and using guards weapons for the executions," he told a news conference on Sunday afternoon. "The criminals carried out their executions as part of a settling of accounts with members of a rival organised crime gang. But they also killed several innocent civilians, in a cowardly manner, before returning to their cells."
Mr Najera added that four prison officials, including the director, Margarita Rojas, and the security chief, are now being held under a form of house arrest while detectives continue their investigation into the alleged crimes. Investigators are questioning all the prison guards, together with roughly half the inmates.
The case, which has dominated the weekend news in Mexico, provides a graphic illustration of the extent to which well-funded drug cartels have managed to permeate the country's apparently corruptible officialdom. Huge profits, of roughly 3,000 per cent per delivery, are being made by gangs smuggling cocaine from South America, where it is produced, to the USA, where it is mostly consumed. The billion-dollar trade has given criminals vast resources with which to bribe police chiefs, politicians, soldiers and now prison guards.
In northern states such as Durango, a recent spike in violence – in which the hotel massacre was the latest major incident – has been attributed to a dispute between the Gulf cartel which traditionally controlled local smuggling routes, and a breakaway gang known as the Zetas, who once worked as their "enforcers".
The discovery that foot-soldiers for the rival cartels have been able to continue their activities from behind bars represents a blow to the credibility of the Mexican President Felipe Calderon's so-called "war on drugs", which kickstarted the recent round of violence when it was launched in 2006. Many important drug barons have been arrested and imprisoned during the course of Mr Calderon's campaign, in which the army has been brought in to investigate and arrest gang members. This weekend in Ciudad Juarez, now one of the most violent cities in the world, police captured Luis Vazquez Barragan, a top member of the La Linea gang, who was wanted on suspicion of several recent murders and a car bombing.
Removing gang leaders from the streets has not so far put a stop to the drug trade, however. Instead, it has merely prompted increases in violence as would-be successors fight amongst each other for the highly lucrative cocaine routes that remain. Even before the scandal at Gomez Palacio, the country's justice system has for years been considered hugely inefficient at the business of investigating crimes, catching criminals and convicting people. Recent reports estimate less than two per cent of crimes reported in Mexico actually result in prison sentences.
Being incarcerated does not by any means lead to an inmate being cut off from the outside world, either. If you drive past prisons in most of Mexico's towns and cities, you will see crowds of visitors arriving at the entrance carrying supplies of food and other creature comforts for relatives who are inside.
As a result, Mexico's prisons are safe havens for many criminal groups. Prisoners have been caught running telephone extortion rings from jail. Escape attempts in which prison officials are in some way complicit remain common. In May 2009, 53 men walked out of their cells at a prison in the central state of Zacatecas, and fled in waiting cars.
The country's Interior Secretary, Francisco Blake Mora, said yesterday that the latest scandal at Gomez Palacio "can only be seen as a wake-up call for authorities to address, once again, the state of deterioration in many local law enforcement institutions". And his government would review "the alleged complicity of authorities so that the criminals, instead of being behind bars, leave with impunity, armed and equipped to commit acts as deplorable as the one last week," he said. "We cannot allow this kind of thing to happen again."