In January of 2017, we sat in a nondescript hotel room in Orlando Florida and listened while Kate (we’ve changed her name at her request) calmly described how in 2014 she was recruited and trafficked out of a state prison by a convicted sex offender and self-styled “super-pimp” Richard Rawls.

After being promised a relationship and somewhere safe to stay, Kate instead left one prison for another. She spent months locked inside Rawl’s house with a group of other women – all also former inmates – who were being controlled with drugs and violence and forced to prostitute themselves to make him money.

It was a shocking story, and one that we would hear over and over again over the following year as we investigated how prisons and jails across the US have become lucrative recruiting grounds for pimps and sex buyers in America’s thriving domestic sex trafficking industry. The work culminated in our documentary, The Trap, which exposes the systematic pattern of grooming and recruitment of women prisoners by pimps and sex buyers in the US.

As two journalists who have covered modern slavery and human trafficking for the best part of a decade, we have travelled far into the dark depths of human exploitation across the globe. Yet for both of us – and for Guardian videographer Alex Healey – this story has been a profound, deeply troubling and emotionally challenging experience.

Kate, featured in documentary, at a park near her home in Casselberry, Florida. Photograph: Todd Anderson/The Guardian

Most of the women we worked with over the last months don’t fit the profile of what many women would think of as trafficking victims. All are American nationals, none have been moved across international borders or controlled by huge organised crime gangs.

Yet across the US, hundreds of thousands of women, men and children are being sold in a multi-billion dollar domestic sex trafficking industry that thrives on the lack of value that society is placing on those being exploited.

All the women we spoke to for this story had personal histories of abuse and trauma. For many, the path into pimp-controlled prostitution had started with a need for love or a relationship, which had been twisted through domestic violence, drug use and basic survival into something very different. None of them had planned or expected their life to end up the way that it has.

We started researching this story at the end of 2015 but didn’t pick up a camera until the middle of 2016 – instead we spent the following year corroborating our initial leads and spent hours on the phone slowly building the trust of the women we wanted to film.

Thanks to the foundation funding that we received to help finance this investigation, we were able to return to Florida, Massachusetts and Chicago a number of times over the months of filming and reporting, an extraordinary luxury in today’s journalistic climate.

As journalists we are supposed to be impartial, using our skills and our platform to report on what is happening to others. Yet trying to process and comprehend the trauma of those whose lives we were dipping in and out of took a toll on all of us.

Despite both of us having a lot of experience on reporting on human rights and trafficking stories, we both found it increasingly difficult to hear story after story of women branded as outcasts of society, who have never had a chance at living a life that hasn’t been shaped by abuse and violence. Most were living hand to mouth on the streets, some were pregnant, many had lost their children. We are both mothers of small children and coming home and having to make the switch from one role to the other became increasingly hard.

For Mei-Ling, the weeks of filming along with Alex Healey inside a jail in Massachusetts, was a disorientating and psychologically jarring experience. Cut off from the outside world, many of the women serving time were only in their 20s but had been in and out of jail and prisons multiple times. Many felt a bleak resignation that things would be no different for them when they were released. There was a suffocating absence of hope.

It was easy to see how these facilities have become hunting grounds for people wanting to exploit this loneliness and isolation. One woman told us that getting a letter from the outside world was like a gift from God.

Then there are the practicalities: without these men sending money into their prison bank accounts, many of the inmates we interviewed wouldn’t be able to buy shampoo, tampons or toothpaste, the things that made them feel human while behind bars. Without these men, few would be able to meet their basic need for housing and food when they left incarceration.

In Massachusetts, we saw first-hand the same women being cycled out of jail and immediately back into a life of addiction and exploitation at the hands of pimps and sex buyers.

That correctional facilities have become recruiting grounds for pimps and sex buyers is just an indication of how pervasive the trap of incarceration and exploitation has become for this community of women. In Worcester, Massachusetts, where some women are finding support, we saw the impact that a criminal record and a lack of exit services or specialist interventions had on those we met in jail. Many felt they had no option but to go back to the same life that put them there in the first place with arrest an inevitability.

The biggest shock was the daily violence that is a byproduct of their life on the streets. For the film we spent a year following Nikki Bell, herself a trafficking survivor who now runs her own NGO, Living in Freedom Together (LIFT), who is dedicating her life to helping other women both inside jail and on the streets of Worcester. She told us how, when she was in prostitution and addiction, she had been beaten, thrown out of cars and violently assaulted by sex buyers. Now she sees this violence perpetrated on a daily basis to the women she works with. Seven women she works with have already died this year of overdoses. One of the most sobering facts we learned on this project was that the average life expectancy of a street sex worker in the US is 34.

The practicalities of working with women who have lived or are living these experiences is difficult and fraught with ethical dilemmas. Insensitive or overly intrusive interviewing can quickly retrigger trauma, yet many of those who agreed to speak to us were desperate for their stories to be heard. Navigating these interviews, which often took hours, was a huge challenge.

What you won’t see is the hours and hours of footage that didn’t make it into the film. The complexity of the story meant not all those people we interviewed could be included in the documentary. Having to explain this to them when they had been so brave to come forward with their stories and so generous with their time was one of our least favourite parts of this whole process.

We also pursued leads which ultimately we had to abandon. In Ohio we followed a sex trafficking survivor who was going through an expungement, a legal process to attempt to get the 102 charges on her criminal record – every one of them the result of her trafficking – wiped clean. After a few days she called us, saying she was finding the whole experience too upsetting and so we agreed to stop our interviews with her immediately.

Yet amid the bleakness, there is also inspiration and hope. Despite all she has gone through, Kate has now left her life of prostitution and addiction behind her and now has a job and a house and is slowly rebuilding her relationship with her family.

The bravery of those who agreed to talk to us and the resilience and compassion of women like Nikki Bell who have survived trafficking and now work relentlessly to try and give others the chance of a different kind of life has been the biggest thing we have both taken away from this story. They can teach us much about how to find light in the darkness of human rights reporting, and we are deeply grateful to them for sharing their lives with us.