How do you identify sex-trafficking victims when such cases go largely undetected or unreported?*

It’s an issue with which law-enforcement agencies in the U.S. continually struggle. Detective Bill Woolf with the Northern Virginia Human Trafficking Task Force has experienced this first hand. Over the course of his tenure, he’s interviewed over 300 victims. In many cases, those who have been exploited believe that they are offenders, Woolf told me. “They fear law enforcement…because they’re technically committing a crime and that is prostitution,” he said.

The Department of Homeland Security defines human trafficking as a “modern-day form of slavery involving the illegal trade of people for exploitation or commercial gain.” In 2012, the International Labor Organization estimated that there are 20.9 million human trafficking victims worldwide. According to the United Nations Office on Drugs and Crime, sexual exploitation is the most commonly identified form ahead of forced labor. Numbers released by the National Human Trafficking Resource Center suggest that also holds true in the U.S., where more than 4,000 cases of sex trafficking were reported. And as a whole human trafficking is a lucrative industry that around the globe rakes in $150 billion.

But it’s unclear whether the numbers are an accurate representation of the problem, because many cases aren’t reported, according to Monique Villa, the CEO of the Thomson Reuters Foundation, which works to combat human trafficking. “The problem with human trafficking is that of course the victims are silenced,” Villa said. “We don’t have good data about it. You don’t know how many slaves there are around the world.” Traffickers also play into the narrative by telling victims, who are exploited for sex, that they are offenders, threatening to call the police and report them for prostitution if they push back. This makes sex trafficking particularly challenging because victims might be fearful of going to law enforcement and being charged with a crime.

It’s a vicious cycle that law enforcement in the U.S. sees time and time again. Women can be pulled in to commercial sex through gangs or pimps—the former function as delivery services, taking women to houses in the area they control, while the latter focus on hotels and street level prostitution, according to Woolf. “In gang-controlled situations, it’s usually going to be that the girl is from the area. When it’s a pimp … it’ll probably be girls from all over the place,” he said.

A woman, who I met through the Thomson Reuters Foundation and who asked that I not use her name to ensure her safety, was pulled in by a pimp when she was 17 years old. Before then, her life was fairly ordinary. She had a good upbringing—a closely knit family and comfortable home. But in high school, she learned that her mother had been embezzling money from her company, and would be sentenced to seven years in prison. That changed everything.

After her mother was gone, she acted out and her relationship with her father fell apart, she told me. So when a guy on Facebook reached out with caring messages, she took notice. “He said everything I wanted to hear, especially with my mom being away,” the woman said. After she graduated high school, the two decided to meet. Then, at 17 years old, she bought a bus ticket to see him, planning to stay with him for a week. But to her surprise, the man, about seven years her elder, immediately told her she needed to make money upon arriving, if she intended to stay with him.

For four days, she said, she worked for him by going to an area for commercial sex. Soon after, another pimp approached her, promising to fill a void—family. She stayed with him for a few months until returning home to see her mother released from prison, after two years instead of seven. Later, she told me, another man courted her on Facebook, asking her to join him in Texas. He also was a pimp.

Unbeknownst to her at the time, the man, who was part of a ring, was luring her in. The woman and up to seven others were taken across state lines to strip and engage in commercial sex. She recalls going to Colorado, Arizona, Indiana, Pennsylvania, and Maryland. When she arrived to Baltimore, Maryland, she had had enough. “That situation was really, really hard for me,” she told me. But leaving was out of the question. If anyone tried to run away, the girls and the men were tasked with stopping them, she told me, adding that she also had no phone and couldn’t be on Facebook. “Literally my rights were ripped from me.”

When the FBI, with the assistance of the Baltimore Police Department S.W.A.T. team broke up the ring, she, too, felt as if she were an offender. “I thought I was getting arrested. I didn’t look at them like they were there to save me. I looked at them like they were there to arrest me,” she said. The FBI had been investigating the ring for two years.

Along the way, victims might encounter law enforcement, as was the case with the victim I spoke with who had been arrested twice for prostitution and, on one occasion, bailed out by her pimp. The U.S. government has pinpointed this as a concern across the country. According to the State Department 2015 trafficking report, some victims, including minors, “were detained or prosecuted by state or local officials for criminal activity related to their being subjected to trafficking” despite “safe harbor” laws in some states intended to protect victims.

But human trafficking cases can be difficult to identify and prosecute. For one, local police may not believe that it is a problem in their community, according to a 2012 study by the Urban Institute. Challenges also surface for state prosecutors, who, the study notes, “were reluctant to utilize new human trafficking laws, commonly charging offenders with offenses they were not familiar with such as a rape, kidnapping, or pandering.”

The Victims of Trafficking and Violence Protection Act of 2000 sought to change that by focusing on the prosecution and enforcement against traffickers. The Department of Justice has cited the legislation after a bump in trafficking investigations and prosecutions. Between the fiscal years 2001-2007, there was a 360 percent increase in convictions compared to the seven previous years.

Despite an enhanced approach from the government, the process can still be grueling for victims. “A lot of these traffickers, particularly on the sex side of things, have developed dysfunctional relationships with their victims,” said Judge Hiram Puig-Lugo, a family court judge in Washington, D.C. Puig-Lugo arranges and oversees services for victims, while also placing emphasis on awareness. In his efforts, he collaborates with NGOs, among them FAIR Girls, an organization that also works to combat human trafficking.

The woman I spoke with is with FAIR Girls now, working as a residential counselor for the organization’s Vida Home, a transitional home for victims. The leader of the ring that she was involved in was sentenced to 17 and a half years in federal prison. But three years later, she still thinks about her ordeal. “It’s been a tough journey,” she said. And it continues to be for law enforcement as they work to identify victims who are afraid to identify themselves.

* This article originally stated that there are an estimated 1.5 million trafficking victims in North America. That figure actually represents the number of estimated victims in North America and Western Europe. We regret the error.