Shortly before noon on Christmas Day 2010, emergency-response workers received a telephone call from a distraught woman asking them to rush to her flat in Newham, east London.
"You must come," the woman sobbed. "My brother has drowned himself in the bath."
As the paramedics raced towards the 14-storey block of flats on Hathaway Crescent they might have been forgiven for thinking they were attending a tragic Christmas Day accident. But within minutes of their arrival they soon realised the scene in front of them was anything but that.
In the bathroom lay the lifeless body of 15-year-old Kristy Bamu. His older sister Magalie, the woman who had called the paramedics, and her boyfriend, Eric Bikubi, kept insisting that the teenager had drowned in the bath. But even a cursory glance showed that Kristy had been horrendously assaulted before he died.
His body, pathologists later found, had 130 separate injuries. Two of his teeth were missing and he was covered in lacerations. The bathroom floor and adjoining sitting room, meanwhile, were smeared with blood.
What the paramedics had actually stumbled upon was a torture chamber where a teenager was brutally murdered because his attackers believed he was a witch.
"I've seen a lot of things and like to think I'm hardened to most situations," one officer who was involved in the investigation told The Independent. "But what happened in that flat shocked me to the core. What those two did to that kid was horrific."
Speaking via a French interpreter, Kelly Bamu, 21, told jurors how Kristy had wet himself and tried to hide his underwear in the kitchen. When Eric and Magalie discovered what had happened they began to accuse him of being a witch. "That is what triggered everything," she said, staring intensely at her sister in the dock. "All over a pair of pants." Soon Kelly and her younger sister were also accused of being witches.
What followed over the next three days was a horrendous attempt by Magalie and Bikubi to "force the Devil" out of the three siblings. The rituals began with long bouts of praying and fasting but soon descended into awful violence as an array of weapons were used on the three hapless victims.
Magalie rarely looked up during her sister's evidence. Giving her own testimony, she argued that she was forced to take part in beatings by Bikubi, whom she described as "controlling" and sometimes violent. But in the end the jury decided that she was just as responsible for Kristy's murder as her boyfriend.
The court heard how Bikubi, a heavily built man, had previously accused a flatmate of being possessed by kindoki, a prevalent belief within Congolese culture that people can be overcome by evil spirits or practise malicious sorcery.
Only the neighbours gave any hint that something was amiss in the flat during the Christmas period. A number of them had complained that they could hear endless chanting and singing but when they tried to knock on the door they were either ignored or told that the family was praying.
Had the police been called, the outcome might have been very different. Instead, on Christmas Day morning, Bikubi and Magalie walked into the sitting room and ordered everyone to shower. Kristy, Kelly and the two younger siblings were pushed into the bath and hosed down. Kristy's battered body couldn't take any more. He slumped to the bottom of the bath and, unable to lift himself any further, drowned. It was only when he stopped moving that the violence against his two sisters finally came to an end.