He and a colleague also helped establish a three-month distance-learning course for officers, part of which trains them to assume the identity of a 14-year-old girl being “groomed” online—the process by which perpetrators steer ostensibly casual interactions toward sexual exchanges or encounters. On top of that, Grant’s work helps inform officers who take on the roles of the image and video traders themselves, trying to sneakily gather intelligence on whom police can ensnare next.

Those who want to abuse children have long been locked in a technological arms race with law enforcement, with tactics including hidden IP addresses, offshore servers that host illegal forums, and the meticulous social-engineering trickery. “What these people have done is developed very sophisticated ways of hiding themselves,” says David Shemmings of the Center for Child Protection at the University of Kent. “I don’t know whether they are actually one step ahead of the security services, but they certainly believe they are.”

Grant is the trick up law enforcement’s sleeve to make sure officers keep up.

Studies carried out in the U.S. and Europe over several decades indicate that up to one in eight girls and one in 10 boys have suffered sexual abuse. The numbers appear to be rising: In 2004 to 2005, there were 1.7 child sexual offences reported per 1,000 children in the United Kingdom, including instances of both physical abuse and indecent photos; by 2015 to 2016, that had risen to 4.9 per 1,000. The number of defendants who were prosecuted for offences involving images of children in the U.K. rose from just over 1,000 in the year 2000 to 3,500 in 2016.

This trend can be attributed to growing awareness of child exploitation and willingness to speak up about abuse in recent years. But that like doesn’t explain all of it. “The reason we’ve seen an increase is perverts and abusers have recognized what a relatively easy way it is to inveigle their way into the lives of our children” by using the internet, says Peter Saunders of the U.K.’s National Association for People Abused in Childhood.

In response, vigilante groups of “pedophile hunters” have sprung up across the world. The American organization Perverted Justice has worked with NBC on To Catch A Predator. In the U.K., some of the fly-by-night groups include Dark Justice. Whether these groups help or hinder the policing of child sexual offences is hotly debated, but their unofficial work does have an impact: Evidence collected from vigilante groups was used to charge 150 suspected offenders in England and Wales in 2017, up sevenfold since 2015.

Still, the scattered groups across the globe taking justice into their own hands aren’t the frontline of the official forces battling against the rising tide of child sexual abuse and exploitation. In the U.K., at least, that’s the police. And the police know what they know in large part thanks to Tim Grant.