Yessenia, 21, spent a year and a half in prison in Mexico City.

She wasn't confined to a building with cells, a yard and guards.

But rather to a street in a bustling blue-collar neighbourhood called La Merced, with her pimp/boyfriend, his family and a network of street vendors and other spies watching her every move.

She worked as a prostitute on the same corner where her pimp/boyfriend's mother, who claimed to have witch-like powers, sold clothing.

Yessenia was terrified.

"I don't know a lot about Santeria, but I know that she can use it to do a lot of things," she says. "She could provoke an accident or kill someone with these types of things."

There is a popular myth that human trafficking begins with a kidnapping and involves physical abuse and confinement.

But experts say that's rare, and the problem is actually much more complicated.

"The myth that the victims are chained in some dark dungeon needs to be broken because it's not the case," says My Lo Cook, strategic initiatives director for Mexico at the Polaris Project, a U.S.-based organization combating human trafficking.

"There are many other methods of control."

Human trafficking, as defined by the United Nations, actually includes any use of deception or coercion — not just physical — to control and exploit an individual.

And in the sex trade in La Merced, that control and exploitation can be a family business in which every member has a role.

Family business

Cook says the men in charge often spend time courting the recruits, who relocate because of love and the promise of a better life.

Boys are raised to covet the large houses and fancy sports cars of their fathers and uncles, and are taught the ins and outs of the family business from as young as eight or nine years old.

And mothers, many former trafficking victims themselves, help normalize the behaviour for new recruits.

Yessenia says she started doing sex work in Acapulco when she was 17.

After police brought her to Mexico City for her own protection following a raid, she reached out to a friend who connected her with someone who could get her out of custody.

The man pretended to be Yessenia's uncle to secure her release. Soon after, they fell in love and he eventually became her pimp, she says.

"In the beginning, it was very different. I liked how he treated me," she says. "We lived together. He was my boyfriend."

But Yessenia says he began exerting more control and demanded more money from her. She soon realized he wasn't who she thought he was.

"I felt imprisoned. I could only go from the house to the street and the street to the house."

Erika, a sex worker who didn't want to give her last name, stands for a picture in La Merced. More than 20 million people live in the Mexico City metropolitan area. (Jonathan Levinson)

Cook says the complex web of relationships makes identifying and combating trafficking difficult.

The Mexican government identified 1,814 trafficking victims in 2015, the last year for which data is available. Of those, 784 were for commercial sex. Federal and state officials initiated 665 investigations but only saw 86 convictions.

But that data is incomplete, as many state and federal agencies do not collect or report complete information.

Network of control

According to outreach worker Letty Cruz, the women and girls selling sex in La Merced — by some accounts, the largest red-light district in Latin America — are almost exclusively Mexican and predominantly from Guerrero and Veracruz, among the country's poorest and most violent states.

Prostitutes dot the landscape at all hours of the day and night. Women and girls stand in doorways and storefronts or under umbrellas to protect themselves from the midday sun. They range in age from young teenagers to women in their 70s.

So, many of these people selling things work for the pimp and they just watch over the girls. - Letty Cruz, outreach worker in La Merced

Standing in her small office space overlooking a pedestrian street crowded with vendors, Cruz, co-founder of a neighbourhood outreach organization called Dunami, explains how the local sex trade works.

"That building on the corner is a hotel," she says. "And the next one is a building where one pimp's entire family lives. So, many of these people selling things work for the pimp and they just watch over the girls."

On any given day, you can find Cruz and her colleagues walking around the neighbourhood handing out condoms and talking to the women. Their goal is to establish a relationship with them and, should any of them decide to leave their pimp, offer assistance.

Halcones and madrotas

Out on the street, even brief exchanges can be dangerous. On a recent Tuesday afternoon, Cruz and her colleague, Gracia Luevano, were out distributing condoms to sex workers.

During every interaction, the watchful gaze of halcones (literally meaning falcons, people whose job is to be the eyes and ears on the streets) was palpable. Street vendors, police, and neighbourhood teens loitering on the corner all serve as halcones, Cruz says. But so do some of the more experienced women, known as madrotas, who also play a kind of managerial role.

While Luevano was talking to one girl, an agitated man standing a few feet away was listening to their conversation.

He creeped closer until finally, scared of what he might do, Luevano had to leave. The man approached the girl, came in close, and could be seen reprimanding her as he went through her purse.

From left to right, Fanny Sugia, Betsy Alfaro, Gracia Luevano and Jazz Sanchez prepare condoms to hand out to sex workers in La Merced. (Jonathan Levinson)

Such interactions are scary but they also help Cruz and Luevano to identify key players in the neighbourhood.

"We need to gain the girls' trust," Cruz says. "If they see us constantly, they start to realize, 'Oh, they're interested in me.'

"I have their numbers and then they start sending messages like, 'How are you?' or 'I don't feel OK, can we talk?' Until finally they share their story with us."

Last month, Yessenia was 900 pesos short on her weekly payments to her husband. Scared, she decided she had to escape La Merced. She sent a text message to the one person she trusted to help, Letty Cruz.

Yessenia managed to relocate safely to another part of the country and land a stable job.

"I feel peaceful, happy," she says. "I'm free."

But back in La Merced, Cruz and Luevano say they've met three new girls selling sex on the streets.

All three are underage.