Two human heads left in a cooler box on the plaza? A mystery. The 18 houses burned in a single night? An enigma. The doctor and his family who disappeared?A rumour.

Juarez valley, which stretches along the Rio Bravo on the United States border, used to grow cotton, residents tell you. But when asked why it has become one of the deadliest places on the planet, they say little. Most refer to “the situation” or, simply, “it”.

Manuel Robles, the curator of the valley museum in San Agustin, can talk about dinosaur fossils but not the present. Pressed, he rubs rheumy eyes, gazes out the window and falls silent. Finally, he says: “If I tell you, tomorrow, I won’t be here.”

It’s as close as you get to an acknowledgement that this valley of a dozen villages, once home to 20 000 people, has detached from Mexico and entered a realm beyond any map. There is no state here, no rule of law. There are killings and beheadings and burnings that no one sees.

The official explanation is that the Sinaloa cartel is challenging the homegrown Juarez cartel for the gun and drug trafficking route to the US. More than 500 people have died here in the past four years, a per capita toll far worse than Iraq. Nationwide 28 000 have died.

As violence raked up and down the valley, exterminating entire families, an exodus began. By the time a church was torched and anonymous notes warned of a bloodbath, most were gone, leaving blackened, boarded-up ghost towns.

The cemetery outside Guadalupe, the biggest town, is a desolate place with fresh mounds. “Four in the past week, all young ones,” says Ignacio Montes (66), the gravedigger.

He indicates a family plot: the mayor, Omar Amaya, killed in 2006, aged 33; his father, Apolonio, killed in 2007, aged 59; Omar’s sister, Aglae, aged 29, and mother, Maria, aged 57, killed in 2008.

“They go after the relatives,” says Montes. During a burial in 2008 gunmen ambushed mourners, killing the dead man’s daughter. Victor Luque (53) is Guadalupe’s acting mayor. His predecessor was assassinated two months ago, the town’s fourth murdered mayor.

Urbane and courteous, Luque agrees to an interview. What’s happening in the valley? “I really don’t know.” Who’s doing the killing? “I really don’t know.” Who’s responsible for security? “I really don’t know.” How many people have fled? “I really don’t know.”

The mayor shrugs, smiles. He knows the exchange is ridiculous. But someone knows a lot about the valley. During a tour, there was barely a soul in sight. Yet the next day, the guide’s family received an anonymous phone call detailing our entire itinerary, who we met, what we discussed.

Ten years ago Mexico completed a velvet transition to democracy after 71 years of one-party rule, with the opposition winning an uncontested victory in presidential elections and the economy growing at 6.6%.

Poverty still affected half the population, there was acute inequality, lingering guerrilla conflict and a worrying number of kidnappings. But there was also an expectation that things would get better.

Today that optimism has transformed into depression and fear as the country reels from the worst wave of violence since the revolution a century ago. Explanations for the decline differ vastly.

President Felipe Calderon says he had no choice but to go after the drug cartels with all the force of the state because the negligence and collusion of previous governments allowed them to take silent control of swaths of the country.

Most deaths, he insists, are the result of turf wars between criminals. Though he admits that his offensive has triggered intensified violence, he argues this is a sign that they are self-destructing under the pressure.

Others link the mayhem to the demise of the semi-authoritarian one-party rule that, however corrupt, set limits on organised crime. “In the old days, the correlation of forces favoured the state,” says drug trafficking historian Luis Astorga.

“Today the struggle for hegemony among drug traffickers is taking place without a referee.” Some analysts argue that the main change has been the transformation of Mexican drug traffickers from the lackeys of Colombian cocaine cartels into the most powerful criminals in South America.

For yet others, the key trigger was a series of high-profile arrests beginning in 2002 that destroyed the underworld equilibrium. The violence will subside, they say, once a new one is established. Many critics of Calderon’s offensive argue that his reliance on military tactics has made things much worse.

The failure to pay as much attention to money laundering, political corruption and poverty has, they say, triggered more violence and encouraged the cartels to penetrate ever deeper into society. Organised crime expert Edgar Buscaglia, a leading critic of Calderon, calls it the “Afghanistanisation of Mexico”.

In recent months Calderon has tacitly admitted that his strategy has holes and has begun calling for the creation of a broad consensus on the best route out of the horror. Juarez valley points to the president’s profound failure, says Gustavo de la Rosa, human rights commissioner in Juarez state.

“It’s abandoned, a land without law.” One reason, he says, is a lack of political will. “There are few votes so politicians ignore it.” The state is present in the form of the army, which has cameras and checkpoints with sandbags on the only road in and out.

But why did they watch thousands of residents flee — convoys of furniture-packed trucks — and do nothing? If there is a pattern, it is that Sinaloa is exterminating suspected Juarez cartel members and their relatives.

Rocio Gallegas, the editor of Juarez’s main newspaper, El Diario, says “it is not possible” that the security forces are ignorant of what is happening. They did catch Jose Rudolfo Escajeda, the Juarez cartel’s valley enforcer, but a Sinaloa commander, nicknamed Quitapuercos “pig killer”, is believed to remain free. It suggests, say some, that the army is tacitly backing Sinaloa.

A similar pattern emerges in Juarez city. On the surface things look normal: the shops and schools are open, there is rush-hour traffic and restaurants are packed at lunchtime. The scythe, however, is busy. More than 6 000 have been murdered since 2008 in a city of just 1.3-million.

Last month was the bloodiest yet: 363 dead, according to El Diario. The city is ebbing away. Many offices and houses are empty, with “for sale” signs outside. About 10 670 businesses — 40% of the total — have shut.

A study by the city’s university found that 116 000 houses had been abandoned and 230 000 people had left. Juarez is the main gateway between Mexico and the US. Railways and roads converge here, as do smugglers.

“Poor Mexico, so far from God, so close to the United States,” observed dictator Porfirio Diaz in the 19th century. With the US the world’s biggest market for illegal drugs, the quip still holds. The absence of security forces in the city breeds insecurity — but so does their presence.

They prey on the population, kidnapping and extorting in cahoots with criminal gangs, according to multiple complaints filed with the human rights commission. In a recent poll 39% cited official corruption as the main driver of violence.

Narco-trafficking — despite government claims — was cited by a mere 14.6%. It is a disturbing finding. The boundary between warring criminal groups and the state blurs and shimmers in the desert heat. Soldiers, police and elected officials fight with and against criminal gangs.

“Our security forces are infiltrated and there are links between criminal groups and certain politicians,” says De la Rosa. ‘They strengthen one another and the phenomenon is getting worse. Some politicians flaunt their connections.”

With his white beard and ponytail, the commissioner resembles a hippy Santa Claus, but is a tough, shrewd operator. He investigates human rights abuses with a small team of young assistants; one of the few state agencies credited with working as it should.

For protection, he sleeps across the border in El Paso and travels to Juarez every day with 12 bodyguards. When thousands of soldiers were deployed in 2008, the violence briefly abated. A city hall source smiles at the memory. “It was a cleaning. And it worked.”

He means that death squads took out mid-ranking narcos, including crooked police. The campaign has never been admitted to officially. ‘But the cleaning stopped after a few months,” says the official. “That was a mistake.”

The authorities did not anticipate how quickly criminal gangs would rebound and co-opt security forces, he says. Police, who have replaced the army on the streets, are seen as ineffectual at best, predatory and murderous at worst. Business owners who spoke anonymously accused officers of treating the city as booty.

“If you don’t pay, you risk disappearing,” says the manager of a car showroom. Municipal and state police are still regarded as loyal to the homegrown cartel, a decades-old tradition. Federal police, outsiders brought in for the drug war, have become linked with the Sinaloa interlopers.

Last month 250 officers roughed up and arrested their own commanders, accusing them of siding with narco-traffickers. A mutiny of the honest, say optimists; a row over cuota, the levy the force charges civilians, say others. Arrest statistics fuel suspicion of favouritism.

Of 81 128 drug-related arrests until the end of July, just 24% were from Sinaloa, the oldest and mightiest cartel. The motive — apart from pay-offs — would supposedly be to end the turf wars by promoting one cartel’s hegemony.

Calderon indignantly denies favouritism, but the Juarez violence suggests local commanders — with or without Mexico City’s approval — have cut deals with Sinaloa. The Juarez cartel, fearing extinction, has lashed back at the black-uniformed federales. An urban guerrilla onslaught has killed about 40 officers since April in drive-by shootings, kidnappings and car bombs.

On a scorching morning recently Bulevar Ampliacion Cuatro Siglos revealed a new cartel tactic: it started with a bloodied, naked foot, continued with chunks of leg, then a trunk, then arms, hands and finally, 200m further on, a head on the bonnet of a Nissan Xterra.

With the quartered remains of federal police officer Hector Mendoza Guevara (25) was a placard: “This is what happens to those who help Chapo.” Joaquin “El Chapo” Guzman is the Sinoloa boss. The grisly tableau seemed designed to instil terror in young officers from southern Mexico, where superstition and belief in sorcery are common.

In Juarez good news passes for this: the federales are so busy trying to stay alive that they recently suspended their extortion rackets, say business owners. The force’s spokesman declined interview requests.

Each day brings fresh horrors to the dystopian city. Two men stabbed and left to die in a dump. Six people incinerated in a van. Two cyclists gunned down on the street. A child shot on the family porch. That was just in one day — before lunch.

Says Charles Bowden, the author of Murder City: Ciudad Juarez and the Global Economy’s New Killing Fields: “In Juarez you can’t see a pattern any more. Killings are everywhere. They cross all class lines.”

There are an estimated 500 gangs in the city, many drawn from slums where parents work in sweatshop factories paying $40 a week. Some are independent, some work for the cartels, some work for the police and some have no idea who they work for. They just take orders over the phone from unknown bosses.

Says Bowden: “It’s like a war in which no one remembers how it started. No one controls the killing now; it’s got a life of its own.” —