I hunt paedophiles throughout the globe on the darkish net utilizing the tiniest particulars to determine them. This is the case that retains me up at night time… and why issues are getting worse

The home video was just a few ­seconds long; a little girl, playing in her bedroom. No name, no location, just a few words in ­English, but not enough to determine the child’s nationality.

Nothing – at least not obviously – ­identifying at all.

Less innocuous were the words of the ­anonymous individual who had posted the footage in the murkiest corners of the hidden layer of the internet known as the dark web.

And how chilling those words were. They went something like this: ‘This is my next girl.’ The words of a child abuser spoken to his own twisted community of like-minded deviants.

To Alex Bromley, sitting at her desk in a police building in Surrey last summer, it was a piece of footage that – even against the backdrop of her own work exposing the depravity of paedophiles – was gut-wrenching to watch.

Here was a child not being abused but one about to be. One with a metaphorical target pinned on her young head – and an invisible countdown clock ticking, somewhere.

‘When you are told a child is imminently going to be abused, working hours go out of the window,’ says Alex, one of only a handful of child sexual abuse dark web investigators – professional, international ‘paedophile hunters’ – in the UK.

If there were a stereotypical image of someone who carries out this vital but harrowing work, it would not be Alex, a civilian Surrey Police employee, who looks considerably younger than her 35 years.

But Alex is steely, compelled to do what she does because it is so important and also because the dark corners of the web are becoming even darker.

When US investigators shared this particular footage with Alex and a band of 20 or so of her colleagues around the globe last year and asked for help to identify both offender and the intended victim, they did what they always do in these circumstances.

‘For me it was two, three in the morning and we just couldn’t let it go, because that child’s life is in your hands,’ says Alex, who has become something of a pioneer in this domain, since training with US law enforcement five years ago.

She adds: ‘You don’t want to sleep. You just cannot stop because you know this child is in danger. At that point she hadn’t been abused; we could potentially save her. We have this video and you feel so close to being able to save her, but she’s behind a screen and there’s nothing you can do.’

The work of this team of investigators was thrown into the spotlight last month when a new Storyville documentary, The Darkest Web, aired on BBC Four.

A crew spent five years filming the work of Alex’s US colleague, investigator Greg Squire, and his specialist contemporaries around the globe.

Compelling viewing, it documents the case of another little girl with shocking similarities to the child on Alex’s computer screen last year.

That was in 2014, when they discovered pictures of a 12-year-old, whom they called Lucy, being distributed on the dark web.

Further investigations revealed the abuse had been going on since she was seven: five long years of unspeakable cruelty and depravity.

The story of how ‘Lucy’ came to be found and rescued reads like an extraordinary jigsaw puzzle.

Abusers are usually meticulously careful about erasing clues to their identity or location from footage.

However, the investigators were able to work out from the electrical sockets in Lucy’s bedroom that she most likely lived in the US, and also identified the brand of the bedroom furniture, including a sofa, which narrowed the search area further.

But in the end, it was a piece of exposed brickwork that solved the puzzle, by narrowing down the specific brick to a ‘Flaming Almino’, a type manufactured in Texas.

As bricks are not normally transported long-distance, being so heavy, it shrank the search area to a 50-mile radius of the factory.

Inquiries to the sofa retailer and access to its customer list helped narrow it down to a list of 50 people.

Finally, following a search on ­Facebook, the investigators spotted what they had been looking for: a picture of Lucy.

She was living with her mother and her mother’s boyfriend, a convicted sex offender who was arrested that same day and is now serving a 70-year sentence.

Although Alex wasn’t a part of that hunt, it’s that same eye for the ­tiniest details, that she deploys every day. ‘There’s so much,’ she says. ‘It’s furniture, it’s bedding, it’s clothing, it’s tattoos, it’s anything you can research and bring back to a particular area. It’s usually not just one object, it will be a variety of things.’

The case last summer, in which a two-week period became a race against the clock to find a target for abuse, was no different.

Of course, if this was the regular internet, computer forensic experts like Alex would be able to trace IP addresses – a unique numerical label attached to every device on the computer network – but, as she says: ‘The dark web masks all that.’

Created in the 1990s by the US Department of Defence so that spies could operate in secrecy, the dark web became something entirely different from about 2004 and has grown into a Hydra-like, sometimes impenetrable, beast where users can hide under a cloak of anonymity.

It’s estimated that its child abuse forums have more than one million active users.

It was here that the video of the girl was posted, and Alex and the team sprang into action.

‘Initially we were sent a video which was only a very small clip, we had very small clues to work from. We all picked apart pieces of the image to work out if there was anything we could identify,’ says Alex.

Ultimately, it was a forum username that set the ball rolling. Someone using the same username had issued an appeal elsewhere online, asking for someone to manipulate an image of a child relative of his.

That username led to an actual name which enabled investigators to comb the web looking for anywhere where that name, and a photograph of this child, came together.

Social media – and hours and hours of forensic trawling, as is so often the case – provided the answer.

The first man was identified and arrested, leading to a second man, in an entirely different country who had effectively been ‘touting’ the child.

The girl was rescued just in time.

‘We found out that we had saved her just before she [could be] abused,’ says Alex, for whom moments like this make the ­relentless horror worthwhile.

An additional satisfaction was that the investigation led to the discovery of a separate, UK-based offender ‘who would never have been found had we not been involved in this job’, says Alex.

So, offences in three countries from one video. It’s why Alex doesn’t feel tied to Home Counties boundaries.

‘This is a global issue and we need to be in that fight,’ she says.

It was a self-funded course that enabled her to get a foot in the door as a forensic computer analyst 11 years ago. She then became an investigator in the paedophile online investigation team and it was in that role that she heard ‘whispers about the dark web’ and ‘how bad it was’; so she cajoled everyone she could – in the US and the UK – to let her pursue more specialist training.

‘It didn’t sit right with me that it wasn’t being investigated thoroughly because I knew how horrendous the content was on there,’ says Alex. ‘I knew the risks posed to children were just so significant.’

It’s an extraordinary and burdensome passion for anyone to carry; within the first 18 months of taking on her role in 2021, 50 child sex offenders across Surrey and the wider UK had been identified.

Greg Squire, a father of two from New Hampshire, talks frankly on the BBC documentary about the impact work like this had on his mental well-being. He describes it as ‘like drinking poison’.

Yet, Alex knows the poison needs to be absorbed if there is any hope of an antidote. ‘These forums are a breeding ground for evil. It’s vast… when one site goes down, another one pops up,’ she says.

‘For every one of us, there’s probably 1,000 of them.’

Abusers fuel each other’s desires. ‘You get one person who starts talking about what they want to carry out, and then they feed into each other’s depraved fantasies,’ says Alex.

The nagging fear, of course, is always that the fantasy might become reality. Alex recalls one case, in which a forensic examination of an offender’s devices led her to a terrifying photograph.

Alex is accustomed to the worst when it comes to images. But a series of pictures here stood out – of two ‘very young’ children made to pose naked on a bed in ­sexualised poses.

But there was a clue. Alex found digital information embedded in the photo that included a man’s name. A Google search revealed he was a photographer.

Alex managed to establish a link between the serial number of the camera used to take the dark web images with a camera used on a Flickr account. ‘And it was just like that moment of, “Oh God, this could be the guy that’s abusing these children,” ’ she says.

The man was in the US, so Alex alerted her peers stateside and soon heard they had linked the same man to further child abuse images. The offender had previously abused children in another country too; so three countries and five children identified and saved from further abuse so far, with enquiries ongoing.

For Alex this underlines the importance of what she does.

‘If my force hadn’t been so supportive and allowed us to pioneer, that guy, I believe, would not have been found because those images were six years old at the time and had gone undetected for that entire period,’ she says.

And sadly, remaining undetected is a feature of this world. Advancing technology helps the hunt, but aids the offenders too. ‘A lot of these people are ­incredibly techie,’ says Alex. ‘Children are learning [programming] in schools, they’re learning coding. So, you’re now getting a generation that is so far advanced; you’ve got AI at your fingertips. You’re always kind of trying to keep up.’

Humans, however, are not infallible.

‘The people that are using [the tech] are human, and humans make mistakes. And we work tirelessly until we find that mistake that leads us to something.’

The fact remains, however, not every puzzle can be found, least of all solved, which leaves, she says, ‘a scar on your soul’.

Using a laptop, her forensic work station is in a small secure office with barred windows that can only be entered by police staff with elevated access and clearance. Her work involves enduring content that can barely be imagined, let alone described.

Often she gets asked how she does it and never shies away from the truth.

‘It’s very difficult and it does change you as a person,’ she says.

‘It makes you see humanity very differently. But I always say the amount of trauma it causes me to watch doesn’t even come close to the trauma that an abused child is going through day in, day out. You put your own suffering aside.’

She recalls one particular video, and the understanding of the boss who sent her home afterwards.

‘I had to sit there and listen and watch, you know, blood-curdling screams of a toddler. It’s not something that you can ever get used to or desensitised from.’

Boundaries between work and home are important, and there are strict rules about where child sexual abuse material can be viewed. But other work can and does take place outside of working hours – because sometimes the window to prevent abuse is so small.

‘You can’t ever take your foot off the brake. That is why I push through and will work these cases into the night sometimes.’

Alex doesn’t want to reveal too much of her own life, outside of work, but she does share that she doesn’t have children of her own ‘at the moment’ and that she has supportive family and friends and keeps herself active.

She seems remarkably grounded for someone who carries such a weighty burden. And although she’s very modest about what she does, the fact she has delivered dark web training to Interpol reveals how respected she is.

She’s off back to her computer as soon as we finish talking, because ‘it never stops’. As she says: ‘It’s getting worse, so how do you fight that? You’re putting out one fire and five more start.’

• The Darkest Web is available on BBC iPlayer now.