The £15bn drugs war that has cost 7,000 lives in 2 years

Enrique Gonzalez was having breakfast at a roadside cafe when a man got out of a car, walked up behind him and shot him in the back with an AK-47 automatic rifle.

When I arrived on the scene 15 minutes later a crowd had gathered around his strangely contorted corpse, three bloody bullet holes through his white shirt confirming the cause of death.

Gonzalez's violent death was just one of 7,000 in the past two years in a brutal war between Mexico's drug gangs, fighting for control of trafficking routes to the United States and Europe, and the Mexican army and police trying to stop them. At stake is not just control of the cocaine trade but the very future of Mexico, which some now say is quickly becoming the new Colombia.

The gangs make a staggering £15bn a year smuggling heroin, marijuana, methamphetamine and Colombian cocaine to the hip bars and dinner parties of North America and Europe.

"The Mexican state has not faced this grave a challenge to its authority since the Mexican Revolution nearly a century ago," warns David J Danelo, the author of The Border: Exploring the US-Mexican Divide.

Mexico's gangs are now so powerful, violent and rich that US drug enforcement agencies say they have operations stretching from the supply end in Colombia and Peru to trafficking routes through Argentina, Guatemala, El Salvador, the Caribbean and Mexico to dealers deep in the US.

Their brand of utterly ruthless violence is marked by instant death for any kind of betrayal or non-payment. Beheadings and kidnappings are common as is high-level government and police corruption. Just last month, Mexico's Federal drug tzar, Noe Ramiriz Mandujano, was arrested on charges that he had received huge bribes from one of the biggest cartels. A witness said Mandujano received $450,000 (£300,000) each month from the Beltran Leyva cartel in exchange for information on investigations.

The cartel was formed in Culiacan, the capital of the north-western state of Sinaloa and now one of Mexico's most violent cities. The city of just 600,000 people is home to at least two of the most powerful cartels and fought over by two more. It is near the wild badlands of the Sierra Madre mountains, on a major drug trafficking route, and is a money-laundering centre and a front line in the Mexican government's war on the drug trade.

Since January, police say, 960 people have been killed in drug-related violence in Sinaloa state. "We are seeing the Colombianisation of Mexico," said one resident. "We are paying the price for not paying attention to this when we should have."

Of the 3,000 people killed in the state over four years, 90 per cent of them were involved in drugs, but, says Mercedes Murillo, a human rights investigator, the other 10 per cent were innocent bystanders. In one week in Culiacan I came across at least 16 murders. In one incident, five men had been killed in scrub 37 miles north of Culiacan. One had been handcuffed, four had been lined up and shot one by one, a fifth man looked like he had been shot while trying to run away. The father of two of the dead men simply said he had no idea why they would have been killed; the code of silence here is overpowering.

Consuelo Barcenas and Alma Herrera are the mothers of two recent victims. Alma's 16-year-old son, Cristobar, died in a hail of bullets with nine other people when cartel hitmen opened fire on a mechanics shop where the teenager was getting his mother's car fixed.

Mrs Herrera believes the police will not investigate the murder because they are afraid of the cartels. "There is no serious investigation into any of the murders here because the entire system – local government, the police, the economy, everything – relies on narco-trafficking." The mothers are taking their case to Mexico's Federal Congress and President Felipe Calderón; their dangerously high-profile act could carry a death sentence.

Yudith del Rincon, a state legislator from Mexico's ruling National Action Party, campaigns against the traffickers and has been the target of intimidation. "The real problem is the link between the drug-trafficking cartels, the people laundering money and the people building business empires out of that money," she said. "The cash is being used to back politicians who then offer the gangs protection. There used to be a line between us and them. That line no longer exists." The legislator has had threatening messages left at her home and her car has been smashed up twice, but she remains undaunted: "I prefer to keep fighting rather than face a future where you don't stand up against the drug trade."

The problem is not confined to her state. Earlier this month, Mexico's acting federal police commissioner, Victor Gerardo Garay, resigned after a senior aide was reported to have been arranging for drugs to be smuggled out of Mexico City's airport.

Most Mexicans believe many of their senior public officials are in the pay of the cartels. With the police force either involved, undermined or outgunned, President Calderon has sent in the army, declaring an all-out war against the cartels. But the gangs are not backing down.

So far this month there have been another 80 killings in Sinaloa with 13 bodies dumped at one location.

Enrique Gonzalez, killed while eating breakfast, had been a police officer. Manuel Inzunza, a local crime reporter, had known Gonzalez for 20 years as a friend, adding that he had been the head of the city's homicide squad, and an honest policeman. "You know, this is the 10th member of Culiacan's homicide squad to be killed since May. Almost the entire unit has been wiped out. The cartels have targeted the homicide squad because they don't want any of the gang murders investigated. When you see police officers getting killed every day it means there is no guarantee of safety for anybody."

Unreported World: 'Mexico: Seven Days in Hell', Channel 4, tonight at 7.35pm