A team of bodyguards fans out through the three-storey building in central Medellín, calling out "clear" after each room is checked. One gunman remains stationed on each floor; another three guard the building's entrance.

With the area secured, a young man in a designer T-shirt and baseball cap emerges on to the roof terrace, followed by his lieutenant. Javier is a trafficker with Colombia's longest-surviving drug cartel, the Envigado Office, but he describes his work in matter-of-fact terms.

"The Office controls the illegal businesses in Medellín. Its main businesses are extortion, hired killings, the traffic in arms and drugs," he says.

The heavy security is soon explained: Javier fears his cartel – and his home city – may be on the brink of another drug war.

Colombia was supposed to have overcome its bloody history. Over the last decade, the government has pushed leftist rebels back into jungles, overseen the demobilisation of tens of thousands of illegal far-right militia fighters and taken down various drug capos.

Washington foreign policy mandarins such as Paul Wolfowitz have held up Colombia as a model for other countries struggling with narco-chaos, such as Afghanistan and Mexico.

And Medellín, Colombia's industrial heartland, was promoted as the embodiment of the country's renaissance: the murder rate plummeted by about 80% over five years, reaching a decade low of 34 deaths per 100,000 in 2007. Once called the "city of death", Medellín was now open for business.

But the root cause of Colombia's violence – the country's status as the world's biggest cocaine producer – has not disappeared. And Medellín's apparent peace lasted only as long as its underworld was run by one man, through the Envigado Office.

Named after a neighbourhood of Medellin, the Office was originally a group of hitmen acting for Pablo Escobar's cartel. After Escobar's death in 1993, the Office was taken over by a former ally turned bitter rival of Escobar, Diego Murillo, known as Don Berna, who cemented control over Medellín and moved the organisation deeper into drug-trafficking.

The Office will collect on anyone's debt, as long as the creditor is willing to give over 50% of what is recovered. It has its own motto: "Debts get paid – with money or with life."

"Many people know that the government won't act as it should, it won't help the people in what they need. Many people come to us to collect money, debts on cars, debts for drugs, basically anything," says Javier.

In his book The Multinational of Crime: the Terrifying Office of Envigado, journalist Alfredo Serrano writes: "Whenever anyone died, people would say that 'they had got on the wrong side of the Office' – as if this criminal organisation held the power of life itself."

Murillo handed himself in to the authorities in 2005 as part of a peace process with the far-right paramilitary groups, but was accused of continuing to run the Office from behind bars, which eventually led to his extradition to the United States. He was convicted of exporting tonnes of cocaine and sentenced to 31 years in an American prison.

After his conviction, the then head of the US Drug Enforcement Administration said: "American and Colombian communities are safer with the removal of this notorious drug kingpin."

But it would not prove so for Medellín. With Murillo out of the way, a vicious power struggle erupted between his successors. Medellín's homicide rate doubled in 2009, leaving about 3,000 people dead.

"Around 15 close friends were killed in the war. We couldn't go out to clubs, we just had to stay home and not get killed," says Javier.

The factional fighting within the Office came to an end last year with the capture of one of the rival leaders, and since then most of the group has reunited under a new boss, just in time to confront a new threat: one of Colombia's emerging narco-militias, the Urabeños.

The Urabeños sprang up after the peace deal with the far-right paramilitaries. While the main militia leaders were jailed alongside Don Berna, most of the mid-range commanders – those who had been running the day-to-day cocaine operations – were free. Many of these commanders reorganised their old outfits, recruited other demobilised fighters, and returned to drug‑running.

The Urabeños are now a force across much of northern Colombia, bringing a military discipline to organised crime.

"They don't think like average narcos," said Jeremy McDermott, founder of Insight Crime, a thinktank that tracks organised crime in Latin America. "They are extraordinarily political, mixed with deep criminal experience."

The group's power was felt earlier this year when it forced dozens of towns to close all businesses after authorities killed a Urabeño leader. And now they are eyeing Medellín.

The looming battle between the Office and the Urabeños is for control of Medellin's underworld, the vast local market and for positioning to be able to negotiate with the Mexican mafias that ship cocaine to the US.

"Many Colombians are moving over to Mexico to firm up relations," says Javier. "We get our guns from there and they get the drugs from us."

Earlier this month, a city-wide police sweep targeted gangs including the Office, arresting 49 people, including seven due for extradition. Last month, the brother of the current leader of the Office was arrested.

But observers say cracking down on the Office will not be enough to keep the peace in Medellín. Jesús Sánchez, who heads the human rights office for the city's ombudsman, says the local government must offer legal alternatives to the legions of hitmen who would fight any drug war.

"The state must do more than just attack the crime; the state needs a greater presence in the poor neighbourhoods," he said.

Infrastructure in the slums has improved, but Sánchez says the city still owes its young men a historical debt: two generations have grown up in a culture of violence and the easy money of trafficking. "When a young man doesn't find work, he's got the chance immediately to join a gang and get all the money he needs."

Javier doesn't see Medellín emerging from its problems soon. "If this is going to change, people must really want to change. But people always want money and power."