One word and a freckle indirectly led to Richard Huckle's arrest.
Police in Australia and Europe were aware of a paedophile site called the Love Zone hidden in the so-called dark web.
It was protected by passwords, encryption and specialist software. Users were totally anonymous.
The images and videos there were particularly disturbing - showing the abuse of babies and very young children.
Members had to post increasingly graphic material to remain on the site. There were tens of thousands of accounts.
Officers with Task Force Argos in Australia knew the creator of the site used an unusual greeting - the word "hiyas".
After exhaustively trawling chatrooms and forums in the open internet, they found a Facebook page of a man who used the same greeting.
Although the Facebook page was fake, they identified a picture of a vehicle and that led them to a man called Shannon McCoole - a childcare worker in Adelaide.
When officers went through his door, he was actually online running the site.
They took detailed photographs of McCoole's hands. This is where the freckle comes in - one on his finger matched exactly one seen in many of the images of abuse.
In an unprecedented move, Task Force Argos assumed McCoole's identity and took over the running of the site.
Last year he was given a 35-year prison sentence.
One user who stood out was Huckle - given the number of children he had access to and his aggressive attitude.
Using data gleaned from the site and information Huckle had posted on social media and other open internet sites, they identified who he was and where he lived.
Realising he was coming back to the UK for the Christmas holidays in 2014, they tipped off Britain's National Crime Agency.
As he landed at Gatwick airport he was met by officers from the NCA's Child Exploitation and Online Protection command.
On his computer and encrypted drives they found tens of thousands of obscene images and videos.