POLICE were sure they knew the identity of the man who raped a two-year-old girl and filmed the sickening attack but they didn't have enough evidence to convict him - until an expert noticed something about his hands.
In July 2014, police in Manchester, England, arrested Jeremy Oketch, 34, on suspicion of committing the horror attack after his girlfriend found the 55-minute long footage on his computer.
Police believed Oketch filmed himself abusing the toddler on a camcorder in 2013 and 2014.
However, the only parts visible of the man in the video were his torso, hands and penis, making a conviction difficult.
In the BBC documentary, The Hands That Convicted a Paedophile, Detective Chief Inspector (DCI) Colin Larkin said it was some of the most horrific footage he had ever seen in his time as a police officer.
"In 27 years of policing, this is the worst footage I have ever had to see. It showed a vulnerable child, and it made my heart break," he said.
"Watching the footage was awful. But to tell the father that his daughter, who he loved and cherished, had been raped was one of the worst things I have ever had to do."
DCI Larkin described the feeling of knowing he had the right person but not having enough evidence for a jury.
"I knew in my heart of hearts that I had the right man. But in the footage I could not see the offender's face, only a torso," he said.
"It's very frustrating know that I've got the right man but knowing that I don't have enough evidence at this stage to convince a jury beyond all reasonable doubt."
Knowing that he needed more evidence, DCI Larkin enlisted the help of forensic anthropologist, Dame Professor Sue Black, who is an expert in the identification of anatomical features, including hands.
Professor Black has helped solve child sex crime cases all over the world since 2006 using a refined analysis technique that examines the perpetrator's physical features.
The police provided Professor Black and her team with a set of images of Oksetch's hands, which they then compared to the hands in the video.
"There are a number of things that we can look at but there are some that are more important than others," she said.
"We will look for scars because they are quite individual, we will look for patterns of skin pigment, we will look for vein patterns and we will look for the pattern of creases of skin over the knuckles."
Professor Black said one of the hardest things about trying to identify the suspect is not automatically convincing yourself the physical traits are the same, just because you believe they are guilty.
The team avoids unconscious bias by instead looking for clues that suggest the person in the footage or image isn't the suspect.
"All the comparison is done by eye," Professor Black said.
"It's a spot-the-difference-type comparison, that game you used to play as a child. I've got this image, I've got that image. What's the same and what's different?"
At first, Oketch pleaded not guilty to the rape charges, but later changed his plea to guilty after the results of Professor Black's analysis were revealed.
The results were conclusive and "every anatomical feature" of the offender in the video matched Oketch's features.
The 34-year-old was sentenced to 15 years in prison and will be on the Sex Offender' Register for life.
Professor Black is working to use her growing database to figure out the probability of two hands having the same features.
Research are yet to find two hands that are exactly the same, even in identical twins.
Her team is also working on developing software that will be able to compare features automatically.