The findings were enough to charge the girls not only with kidnap, but also with intent to commit a serious sexual assault.

However, after review by the Crown Prosecution Service, the evidence for the latter was not considered strong enough to secure a conviction.

Police were sure Rose had not been sexually assaulted.

The teenage girls pleaded guilty to kidnap, and the sexual offence was withdrawn.

They were each sentenced to three-and-a-half years detention in July 2016.

The judge, Mr Justice Globe, said he was convinced that they had intended to seriously harm the little girl.

“It is not possible to state with any certainty who was going to cause the harm or exactly what that harm would have been except to come to the conclusion that it would have mirrored some aspect of the physical or sexual violence and/or exploitation that was found on the tablet,” he said.

“In my judgement it was the true reason why she was taken.”

The police investigation had only uncovered one other hint to the girls' motives - some, limited, evidence that the 13-year-old had been in contact with a man, using an online chatroom.

It seems the girl had alerted a teacher at her school to the fact a man had asked her to send photos to him.

The teacher took some screen grabs of the chat session, and it is clear that the conversation was of a sexual nature. But then the trail ran cold. There was no evidence the man was involved in encouraging the kidnapping.

The school is said to have alerted the girl's mother, and the tablet computer, along with the teenagers' phones, were confiscated.

The chatroom exchange was at least two months before the abduction in April and by the time the police seized the tablet, the app it had been on had been deleted and all records of the chat wiped.

The teenagers refused to explain any of their online activities to the police, but claimed to others that the man in the chatroom was called Nazzer, and that he had put them up to the abduction.

Police were not convinced. They examined thousands of hours of CCTV, but saw no evidence of anyone suspicious being present during the kidnapping.

The sisters' only explanation for their actions came in the form of a written statement, which repeats claims the little girl followed them and that they intended to return her to her mother - says Det Sgt Stu Liddell:

 We are not dealing with two angelic young schoolgirls. They were devious, manipulative and proficient in lies.”

Looking back on the traumatic afternoon several months later, Rose's mother says she forgives the teenage girls and feels pity for them.

“I think everybody asks themselves that question why? Their parents must ask why? Their classmates must ask why? The judge asked why? But unfortunately that is one answer which has not been given.”