Medellin's hillside slums -- known locally as comunas -- are the fiefdoms of street gangs that not only control criminal activities, but also exert a tight grip on every facet of daily life.

For local youth, contact with the gangs is unavoidable and many get drawn into their orbit. For girls in the comunas, recruitment begins with children as young as ten.

According to one youth worker in the city's troubled Comuna 13 district, who did not want to be identified for fear of reprisals, once the process has begun it is near-impossible to stop.

"It is a chain," he said. "They start with these girls, who get the benefits offered by these structures -- the drugs and the money -- and the girls become emissaries, bringing in other girls."

The girls' families, caught in a cycle of poverty and fear, have little option but to watch their daughters disappear. If they accept the gangs' advances they receive protection and financial help to ease the dire poverty in the comunas. If they reject them then they can either flee their homes or live in fear that every passing motorbike could be carrying an assassin, and every knock on the door could signal another name added to the list of Medellin's disappeared.

Some of these children become concubines for the gang members, a situation now so common that it is accepted as a normal part of life in the comunas, according to Jesus Sanchez, the Medellin human rights ombudsman.

"In many cases the victims don't understand it as something bad, something that goes against morality, against ethics and human dignity," he said. "There is this context where members of armed groups having sexual relations with girls of 10, 12, 13 years, who go with various members of the group, is not looked on as something bad."

However, for others, an even darker fate awaits, as they are passed through the criminal networks of which the gangs are only the street level operatives, and enter the shadowy world of the Medellin sex trade.

For some, this journey begins with their virginities being placed on an auction block.

"[The gangs] seduce her by offering her consumer goods, gaining her trust little by little until they end up being photographed and auctioned off," said Luis Pardo, Director of NGO Centro Consultoria de Conflicto Urbano (C3), which has been investigating the phenomenon over the last year.

Some are offered up to Colombia's top drug lords and paramilitary chiefs, who have kept up the tradition begun by Pablo Escobar of having teenage virgins delivered for themselves or for their narco-orgies. According to Pardo, the girls are passed from one capo to another until they are discarded, left with little option but to turn to prostitution.

However, many of the girls are destined to be auctioned off to rich foreigners. The security improvements of the last decade have led to a surge in tourism as the city's reputation metamorphosed from a no-go zone of drug trafficking and extreme violence, to a thriving cosmopolitan city. The dark side of the influx of tourists has been a boom in sex tourism, with foreigners attracted by the city's reputation for beautiful women and Colombia's lax prostitution laws.

In recent years, sex tourism networks have proliferated. Many are run by foreigners, mostly US citizens, who offer guided tours of the city's brothels and red light hotspots over the internet. Although prostitution is legal if the women are aged 18 or over, the tours are illegal as they are classified as acting as an "intermediary."

According to C3's investigations, it is these networks' most trusted clients that participate in the auctions. Photographs of the girls are collected for brochures -- a small selection for hard copies, or online catalogues of up to 60 girls. Online customers are issued a secret PIN number to access the auction. C3 recorded bids of up to $2,600.

"It is a typical auction," said Pardo. "The highest bidder is sold the girl's virginity -- it is a horror."

Following the auctions, the brochures are destroyed and the auction sites taken down, leaving almost no trace of what happened, according to C3. Most of the girls also disappear, lost to the Medellin underworld.

Many will end up in the city center, where there are several zones notorious for child prostitution.

The gangs that run the city center are known as the Convivir -- a name taken from community self-defense groups set up by former President Alvaro Uribe when he was governor of the department of Antioquia, which descended into paramilitarism and criminality.

As with the gangs in the comunas, the Convivir tightly control both civilian and criminal life, including establishing authorized zones for activities such as drug dealing, street robberies and prostitution.

However, while the Convivir regulate crime in the zone, it is specialist networks that actually manage the sex trade, according to city center residents.

Some of these networks are little more than protection rackets taking a cut of profits, locals say.

"There are criminal bosses, and the children that go to work there have to give them part of what the client pays, if not then they rob them and run them off," said Edal Aldniel Yurient Monsalve Bran, a local resident and community leader.

Others are more organized, trafficking girls, organizing auctions and maintaining properties or links with hotels where clients can go.

"They give [the girls] everything to maintain them, drugs, things for prostitution, clothes -- it is very organized -- and when the police come they pay them too," said one community leader and resident, who did not want to be indentified for security reasons.

The Medellin police did not respond to InSight Crime's request for an interview.

Many of the girls drawn into gang life and the Medellin sex trade simply disappear. In 2013, close to 600 children have disappeared -- a majority of them girls -- according to investigators in the mayor's office. The investigators say this number has been rising sharply in recent years, and although they cannot be sure, they think they know why.

"We believe that the girls that disappear are being recruited for human trafficking or the sexual exploitation of children," said Catalina Alvarez, an investigator with the Medellin Ombudsmen's office.

The Medellin authorities say they are aware of the scale of the problem of sexual exploitation of children in the city, but their ability to tackle it is shackled by fear of the gangs and the silence this creates.

"We have contact with the community, and are trying to build trust so that victims come to us, but going from comuna to comuna looking for them is completely impossible," said Alvarez.

However, those working to protect the girls are critical of their efforts. "The response of the authorities has been minimal," said Clara Ines, Director of Medellin women's rights NGO Vamos Mujer.

"The girls that are used for this need to be able to report it, they need to be able to see the way they can save themselves, the way they can protect themselves -- because they kill them," she said. "The girls are running a really high risk and that is why we are seeing more and more dead girls and women."

However, Ines believes that to truly tackle the issue Colombia and Medellin must also confront an uncomfortable truth about the roots of the problem -- the "narcotization" of the culture.

"In the narco-trafficking culture, girls are worth nothing," she said. "They are disposable."