So many different criminal groups have terrorised the slums of Colombia’s main Pacific port that residents rarely bother to learn the name of the latest clan in control. They simply call the warring gangs los malos or the bad guys.

The rival factions fight for control of some of the poorest neighbourhoods of Buenaventura, a city of 290,000 people that serves as the country’s gateway to the Pacific and handles about half of the country’s cargo. Many of the barrios are major routes for drug trafficking. They also happen to overlap with areas where the government and private investors are planning big infrastructure projects.

The criminals recruit children, extort businesses, force people from their homes and dismember live victims, scattering their remains in the bay or surrounding jungle. Dozens of wooden huts balanced precariously on stilts over the bay have been abandoned by terrorised citizens and taken over by the gangs for use as casas de pique, or chop houses, where they torture and murder their victims.

The chop houses are the most gruesome consequence of a deeply flawed attempt to dismantle rightwing militias, which originally emerged to combat leftwing guerrillas in collusion with state security forces and drug traffickers. These paramilitary groups were gradually demobilised from 2003, but many former fighters neither went to jail nor joined the reintegration programmes, choosingto by the gun as part of new criminal groups.

The Colombian government has been undertaking a fresh round of negotiations – this time with the leftist Farc rebels – in an attempt to end half a century of civil war.

If successful, the talks would eventually lead to the demobilisation of 20,000 guerrilla fighters, but analysts and activists warn that the violence in Buenaventura is proof that a deal will not automatically bring peace.

“What we’re seeing in Buenaventura is post-conflict violence,” said Jorge Restrepo, director of the Conflict Analysis Resource Centre in Bogotá. “It is the application of forms of violence associated with the political conflict – forced disappearance, displacement and dismemberment – to other types of disputes.”

Colombian navy special forces on patrol in Buenaventura. Photograph: Fernando Vergara/AP

The disputes in the city have nothing to do with ideology. They are a mesh of conflicts over land, drug-trafficking routes and extortion rackets.

Since demobilising, at least 24% of former paramilitaries have returned to crime and another 56% are at risk of recidivism, according to the Colombia-based Ideas for Peace Foundation.

This is especially true in areas such as Buenaventura, where criminal activity such as drug trafficking and illegal mining is rife. “If it’s not done right, the risk of former combatants getting swept up in criminal networks is high,” said María Victoria Llorente, director of the Ideas for Peace Foundation.

Violence has been rampant in Buenaventura for decades. The city has some of the highest rates of forced displacement and homicide in the country. But seldom has it been confronted by the levels of brutality experienced in the past year, when a new group, the Gaitanistas, came to town.

In March, authorities dismantled a dozen chop houses where dismemberments had taken place. The government sent soldiers to patrol the most violent areas, but the slaughter has continued. Body parts still wash up under the stilts of homes. “You don’t need a house to dismember people,” said Wilmar Valencia, a community leader in the Viento Libre barrio. “They can do that anywhere.”

A detective recalled being called to the scene of a dismemberment on a Saturday night in June, just after Colombia beat Uruguay to reach the World Cup quarter finals.

“Everyone was celebrating but we got an anonymous call that screams had been heard from a house in one of the poorer neighbourhoods of the city,” said the detective, who asked not to be named.

By the time police arrived, the killers had fled, but they had left the remains of their victim in two burlap sacks. “The place looked like the butcher’s section of a market,” the detective said.

The victim in that case did not make it on to prosecutors’ roster of the disappeared. So far this year, 22 missing person cases have been reported: one was found alive and another found dismembered; the rest are still missing and presumed dead. “Most likely they have been chopped up too,” said one prosecutor. “But we only hear about a small portion of the disappearances.”

Based on body parts found, 39 cases of dismemberment are under investigation. But trying to match the remains to a missing person is “like a jigsaw puzzle”, the prosecutor said.

A fortysomething woman, who asked to be identified as Luz Dary, said her family had reported her 19-year-old nephew missing last year, so when officials found body parts tangled in the mangroves just outside the city, officers got in touch. “They only had the torso and we were able to recognise him by a burn scar he had that covered his whole left side,” she said.

Dary spoke softly as she told her story of terror and displacement, but when a shadow darkened the open doorway she suddenly began talking animatedly about how to make coconut sweets. One of the “bad guys”, a neighbour, wandered in and settled ominously on her sofa.

“The situation in Buenaventura is among the very worst we’ve seen in many years of working in Colombia and the region,” said José Miguel Vivanco, Americas director at Human Rights Watch, which published a report on the violence in the port city this year.

A memorial to victims of violence in Buenaventura. Photograph: Luis Robayo/AFP/Getty Images

The reasons behind the violence in Buenaventura are not easily pinpointed. Officials say it is all about controlling routes for cocaine, Colombia’s biggest illegal export. Local police say there is some evidence that Mexico’s Sinaloa cartel is trying to take direct control of exports. But community leaders say that while that was largely true a decade ago, it is far less the case today. “They don’t move drug shipments through here any more,” said Valencia.

The community leader sees darker interests behind the violence, saying the areas where most crimes occur are the same where plans have been laid for a waterfront project, an airport and seaport terminals. “I see the violence as a means of pressure to get us off this area so they can build their projects,” said Valencia.

Mayor Bartolo Valencia (no relation to the community leader) said the concentration of violence in areas awaiting redevelopment was a coincidence, though he said he was determined to resettle the 100 or so families of the San José neighbourhood in the city centre to make room for a waterfront project to attract more tourists.

Even the “bad guys” are unsure why they are fighting. A young man who asked to be called Longi said he joined the gangs when he was 14. After five years – during which he claims he killed 15 people, including three by dismemberment – he was promoted to mid-level leader.

“The idea is to control all the neighbourhoods in Buenaventura,” he said as he fidgeted with the wooden rosary around his neck, finding it hard to focus in his drug-induced haze. But when asked what the control is for, he answers: “You have to ask the bosses that. We get the orders from the old men, the bosses. They tell us who to kill, who to chop up,” he said. “But I don’t know why.”