Daniela remembers being driven blindfolded through the desert in northern Mexico, thinking she was going to her death. She recalls being told to get out of the van, uncover her eyes, and follow her armed captors into a large house and down into the cellar. She was obliged to watch what was going on, and tried to blank out her mind.

It didn't work. She still remembers the scene — about five young women bound to pillars, surrounded by men who had paid a lot of money not just to rape them, but to torture and perhaps kill them as well.

Daniela is not her real name. She insists on a pseudonym, because she may have escaped, but the reach of her former captors is long. What she saw that day was just one of many horrific experiences she went through in seven years as a sex slave, first under the control of the Zetas cartel and then the Gulf cartel. It ended with her escape last year and her return to her family in Nicaragua, where the nightmare had begun.

"I saw lots of people die, and die in horrible ways," she says, drinking hot chocolate and eating pizza in a Mexico City cafe. "I want to talk because people have to know what is happening on the border to the girls who are disappeared, and with lots of the girls who are working in the sex trade in narco areas."

Daniela's case is now being investigated by the Mexican attorney general's office, but if she had waited for the government to come save her, she would almost certainly not be free today.

'I want to talk because people have to know what is happening on the border to the girls who are disappeared'

According to Mexican government figures, 20,203 men and 7,435 women were classified as "missing or disappeared" in the country at the end of last year. That's pretty much the same number as it was when President Enrique Peña Nieto took office in December 2012, promising to do more to end the horror of those disappearances at the hands of the drug cartels. The pledges intensified after the mass disappearance of 43 student teachers in September 2014 in the southern city of Iguala triggered national, and international, outrage.

Human rights groups in Mexico and outside have repeatedly lambasted authorities for not doing more to find the missing. This week, 68 US members of Congress signed a letter addressed to Secretary of State John Kerry on the "ongoing human rights crisis in Mexico." It began with a reference to the disappeared.

Those who go missing rarely come back to tell their stories. The reasons for their abduction vary, but many are kidnapped into sexual slavery. Daniela's story of survival is as extraordinary as it is harrowing.

Daniela says that she herself was shocked when she discovered the length of her ordeal. She'd estimated it had lasted four or five years, but her kidnappers had always kept her deliberately confused about the passage of time.

"Sometimes when I was with a client I would find out what month or year it was because it came up in the conversation, but if they heard me ask a question they would beat me really badly," she recalls. "There was no radio, no TV, no newspapers, nothing. I slept in one of their houses, they took me to the clients to do ugly things, they took the money, and then they took me back to sleep."

It all began when she was a 22-year-old, struggling to feed her children and her mother by working as a seamstress in an assembly-for-export factory near her hometown in Nicaragua.

It was April 2008 and Nicaragua was relatively free of the horrific violence that already rampaged in parts of Mexico, and in neighboring countries in Central America. Daniela had no obvious reason to be wary about accepting an invitation to attend a meeting in which she was told she would be evaluated for a loan.

All 15 young women who went to that meeting near the Honduras border were kidnapped.

Gunmen took their identification papers, and dressed them in clean jeans, t-shirts, and white baseball caps. They threatened to kill their families if they didn't follow detailed instructions while being driven through border posts into Honduras, and then Guatemala, Belize, and finally Mexico.

Two days into the trip they stopped in the city of Comitán in the southern Mexican state of Chiapas and were put to work in a dark and dirty brothel. Daniela says they were beaten when their inexperience showed. Two weeks later the group headed north again. Daniela says she was the last to be delivered to her new masters, in Nuevo Laredo, in the state of Tamaulipas, a stone's throw from Texas. It was only then that she found out she was in the power of the Zetas.

The Zetas were formed in the 1990s from a kernel of deserters from the special forces, recruited into the Gulf cartel to protect its then-leader, Osiel Cárdenas. By the time they kidnapped Daniela, they were a criminal organization famed for the ruthless way they disposed of their enemies and exercised power over people who lived in their strongholds, the most important of which was in Tamaulipas.

Nothing during the interview affects Daniela as deeply as when she talks about the boy she thought of as a little brother while they both worked at the Danash table dance club in the center of Nuevo Laredo. She met him when he was just 12 years old, and the memory makes her sob.

Daniela was supposed to dance, drink and get high with clients, and fulfill a quota of six sexual services a night in the cubicles of the club. He was a busboy, a messenger, a lookout, and a DJ, who was also rented out to clients, many of them US tourists, who wanted sex with children.

'I saw lots of people die, and die in horrible ways'

Daniela and her putative little brother snatched furtive chats when their captors weren't looking. They fantasized about freedom. They helped each other survive.

When the boy got digestive problems that stopped him from working, cartel gunmen took him and Daniela to the mountains outside the city.

There they gave Daniela a gun and ordered her to kill him. When she wouldn't, they gave the gun to the teenager and told him to kill her. When he also refused to shoot, they hung him from a tree and began to cut him. Eventually they killed him.

"I never heard anything more about him," she says.

Daniela later learned that the order to kill was a test to see if she could be transformed from a sex slave into a sicario — a hitman. When it was clear she didn't have what it takes, the cartel gave her drug smuggling duties instead. That is a recognized pattern, in which sex trafficking victims are pushed into new criminal jobs once they get older and make less money for their captors.

Daniela says that her new job brought her into contact with cartel leaders with names well known to law enforcement, like Z-40, Metro 3, and Catracho.

She also met a Zeta known as La Ardilla, and says she was with him when he ordered the massacre of 72 Central American migrants in August 2010. She later told the authorities that he killed them because he thought they were reinforcements for his enemies.

That massacre came as the Zetas and the Gulf were fighting an all-out war following their split at the start of that year. It included major battles involving very heavily armed gunmen carried in convoys of dozens of vehicles at a time.

Daniela started the war as the slave/lover of a Zeta commander nicknamed El Viejón who had chosen her. When he decided to change his loyalties to the Gulf, she inevitably went with him.

'I thought, well I'm dead anyway, so I let them run and hide'

Being identified as the personal property of a cartel boss meant Daniela had a localizing chip inserted into her foot. It did not, however, free her from the obligation to have sex with clients as well. In fact conditions at the brothel where she worked got even worse once the Gulf cartel took over, despite the Zetas' more bloodthirsty reputation.

Daniela says the new bosses videotaped clients from the moment they entered the bar. The rooms had hidden microphones and cameras.

She says the women were shown videos of the torture and murder of those caught trying to escape. Becoming an addict could also lead to being disappeared for good. Then there were the victims of the clients who paid to torture and even kill them, like the women she had seen in a basement around the middle of her ordeal. Daniela claims her captors once showed her a video of henchmen feeding body parts to a lion kept in a safe house in the city of Reynosa.

Though Daniela says she did her best to stay out of trouble, once she courted death when she was told to watch over a couple who had been kidnapped.

"It was the first time I'd ever been told to watch anyone, and they looked so sad," she remembers. "I thought, well I'm dead anyway, so I let them go. I let them run and hide."

She says that act of rebellion brought her a horrific beating and then a trip to the countryside where El Viejón got into a tractor and threatened to run over her. Then he changed his mind and instead ordered her to spend hours on her knees in front of cartel members, after which she was shut in a van with nothing to eat or drink until near death.

Then, Daniela says, it was back to work at the brothel.

Photo by Daniele Giacometti/VICE News

According to the latest government report on sex trafficking in Mexico, dated 2014, there are 47 identified criminal groups involved in the business, with leaders in Central America, Mexico, and the United States, and bars and discos around the northern frontier.

Daniela is not sure if any of the clients knew she was a slave, but she believes some had their suspicions. She says sometimes it was clear they noticed her bruises — even though she was not beaten in the face, and the cubicles were dark — but they turned away. And appealing for help was out of the question, she says, though sometimes she tried to communicate her desperation with her eyes.

Daniela won't give the details of how she eventually escaped, got the chip out of her foot, and fled to safety. She will only say that somebody risked their life to help her. "They got me out of the place, they paid my transport to Mexico City," is all she will say. "If I say more they will kill that person, and I would never forgive myself."

She has good reason to fear for her savior's life. Violence continues unabated in Tamaulipas even after the end of the war between the Gulf and the Zetas, which has now been replaced by a multiplicity of battles between many different factions of both cartels.

Daniela also won't talk about the details of how she ended up telling her story to federal investigators in Mexico City. Initially, she says, they sent her back to Nicaragua, but her case was revived after the Mexican NGO Unidos contra la Trata, which works on sex trafficking came knocking on her door.

The activists persuaded her to get in contact with a special prosecutor for sex crimes. Her hope, she says, is that the investigation will lead to raids freeing women she thinks are still captive and helpless.

Daniela says that her family filed a missing person's report in Nicaragua when she disappeared. They also went to the local television station, and put up posters. Then they gave her up for dead. She, like most Central Americans who go missing in Mexico, wouldn't even have appeared on the Mexican register of 28,000 disappeared.

Daniela recalls her mother's incredulity when she called from a police station in the Mexican capital. She began to believe her only when the conversation turned to old memories, like the dress she'd made her daughter for her 15th birthday that was too long.

"Daughter, you are alive," Daniela remembers her mother cried when the penny finally dropped. "Yes, I'm here, Mom, I'm here."

Alan Hernandez contributed to this report.