The shootout left four people dead, but that was just the beginning. As dust began to settle on a ranch in north-eastern Mexico, thought to have been owned by one of the world's most powerful drug cartels, the battle-hardened Marines stumbled upon their first decomposing corpse.
Minutes later, they found a second, then a third. By the time troops had finished searching the remote property, roughly 90 miles from the US border, a total of 72 contorted bodies had been laid out in rows beneath the summer sunshine. The 54 men and 18 women had all been recently murdered.
A lone wounded survivor, who was left for dead but later stumbled upon a military checkpoint, told local newspapers yesterday that he and the victims were illegal migrants from Central America trying to make their way to the US. They had been taken hostage by the Zetas, a gang of drug-runners who have recently taken to kidnapping and human trafficking. The Ecuadorian man said his group was taken to a ranch by gunmen and shot after they refused to pay ransoms.
The discovery on Tuesday afternoon marked a new low in a brutal conflict that has taken the lives of an estimated 28,000 Mexicans since the President, Felipe Calderon, declared "war" on the nation's wealthy and extraordinarily well-armed drug cartels in 2007.
Troops originally raided the ranch near San Fernando, in the Gulf coast state of Tamaulipas, after a man with gunshot wounds approached a military checkpoint and said he had been attacked by a narcotics gang. Naval helicopters were dispatched to the ranch but, as they approached, several gunmen opened fire with automatic weapons and tried to flee in a convoy of vehicles. In the ensuing shootout, a Marine and three suspected cartel members were killed.
At the ranch, the Marines seized 21 assault rifles, shotguns and rifles, with 6,000 ammunition rounds. Then they discovered what a spokesman called "the lifeless bodies of 72 people". It was not clear whether the victims were separately, or in a single massacre.
Mass graves are becoming an increasingly common by-product of the wave of drug-related violence sweeping the country. In May, 55 bodies were pulled from abandoned mine near Taxco, just south of Mexico City. Last month, 51 more were unearthed from a field next to a rubbish tip near the northern city of Monterrey.
They provide stark reminders of the growing cheapness of life in a conflict that is constantly plumbing new depths of barbarity. Over the weekend, four decapitated bodies, their genitals and index fingers cut off, were hung upside down from a bridge just outside the nation's capital. Two more were dumped nearby on Tuesday.
"The federal government categorically condemns the barbarous acts committed by criminal organisations," the Navy said of the latest atrocity. "Society should condemn these acts, which illustrate the absolute necessity to continue fighting crime with all rigour."
Tamaulipas, on the north-eastern tip of Mexico bordering Texas, provides a stark illustration of the problems facing the forces of law and order across the country, as they attempt to crack down on gangs smuggling cocaine from South and Central America, where it is produced, to the US, where most of it is consumed.
For years, local supply routes were controlled by the Gulf Cartel, a long-established criminal organisation which kept its activities largely beneath the public radar. But in 2007, shortly after the newly-elected President Calderon announced a crackdown on the drugs trade, several of the group's leaders were arrested. Instead of finishing off the cartel, though, that led to the rise of a rival group, the Zetas. The subsequent turf war has claimed hundreds of victims.
It is also thought to have led to widespread corruption at the highest levels of the police and civil service, together with the murder of Rodolfo Torre Cantu, a popular candidate for state governorship, who was shot dead in his car in June in Mexico's worst political killing in 16 years. Mr Calderon told Mexicans this week to brace themselves for further killings. But he argued that the spate of deaths showed that his crackdown, which has involved replacing often-corrupt police forces with government soldiers in many regions, is slowly working.
"I do not rule out that there might be more bouts of the violence we are witnessing, and what is more, the victory we are seeking and will gain is unthinkable without more violence," he said. "But this is a process of self-destruction for the criminals."
Although most Mexicans support Mr Calderon for now, a growing minority believe that the drugs war will be impossible to win. Earlier this month, former president Vicente Fox, a staunch supporter of the US crackdown on drugs, said recent events had won him over to the cause of legalisation. "It does not mean drugs are good," he said. "But we have to see it as a strategy to weaken and break the economic system that allows cartels to earn huge profits."
To legalise or not to legalise: the drugs war in words
Mexican President Felipe Calderón, June 2010
"It is as though we have a neighbour next door who is the biggest addict in the world, with the added fact that everyone wants to sell drugs through our house... If we remain with our arms crossed, we will remain in the hands of organised crime, we will always live in fear, our children will not have a future, violence will increase and we'll lose our freedom."
Former Mexican President Vicente Fox August 2010
"We should consider legalising the production, sale and distribution of drugs... Radical prohibition strategies have never worked."
US President Barack Obama April 2009
"At a time when the Mexican government has so courageously taken on the drug cartels that have plagued both sides of the border, it is absolutely critical that the United States joins as a full partner in dealing with this issue... also on our side of the border, in dealing with the flow of guns and cash south."
Samuel Gonzalez, former anti-drugs prosecutor, August 2010
"In almost four years the government cannot claim any kind of victory and the debate is the result of the crisis of legitimacy in the strategy. But at least it is now being discussed and that has to be a good thing."