French au pair Sophie Lionnet, in an undated photo. (London Metropolitan Police Service/AFP)

Neighbors spotted the black smoke lifting from behind the house on Wimbledon Park Road in the Southfields neighborhood of southwest London. The smell was so alarmingly pungent and foul firefighters were called to the scene. When the crew pushed into the garden of the $1.2 million residence, a bonfire was belching flames. Once the blaze was doused, firefighter Thomas Hunt’s eyes zeroed in on the ashes: Clothing, jewelry and a human nose and fingers appeared to be in the smoking pile.

“Why are you burning a body?” the disturbed firefighter asked the homeowner, Ouissem Medouni, Hunt would later testify.

“It’s a sheep,” Medouni, a French Algerian financial analyst, replied.

“Bollocks,” the firefighter answered. Later, Hunt would tell a court Medouni then insisted the remains were from an animal but that a look of “resignation” crossed his face, as if “the game’s up.”

Police arrived at the London house that day — Sept. 20, 2017 — to recover what would be later identified as the charred body of Sophie Lionnet, the 21-year-old French au pair hired to care for Medouni’s two young children.

What followed was a murder saga of torture and obsession centering on Medouni and his longtime partner, Sabrina Kouider, a glamorous would-be fashion designer. After a two-month trial in London, on Thursday both Medouni and Kouider were found guilty of killing Lionnet.

According to the Telegraph, the judge in the case noted the crime was a remarkably rare instance of “folie a deux” — or dual psychosis. Kouider, diagnosed with depression and borderline personality disorder, had affected her lover with her own unhinged delusions. The testimony presented at the trial showed the couple submitted the young au pair to starvation and brutal torture until she admitted her part in the fantasy that had twisted both Medouni’s and Kouider’s minds — a convoluted paranoia about Kouider’s ex-boyfriend, a famous Irish pop star.

Kouider, 35, and Medouni, 40, have a long and rocky history as a couple. Born in Algeria but raised in Paris, Kouider was 18 and working at a sweets stall at a fair when she was spotted by Medouni, the Telegraph reported. The older man pursued the young girl and they fell into a relationship. When Kouider left for London to work as a nanny, Medouni came, as well, eventually earning a degree in economics and taking a job with a French bank.

Although the couple was joined by a traditional Muslim marriage certificate, Kouider openly flouted the relationship, referring to Medouni as a relative or friend publicly while dating other men. In court, prosecutors presented Medouni as weak and pliable, open to suggestion, according to the Times of London.

Ouissem Medouni and Sabrina Kouider. (Metropolitan Police/AP)

In 2011, Kouider met Mark Walton at a Notting Hill bank. The Ireland-born founder of the 1990s boy group Boyzone, Walton had gone on to major success in the music industry while working with artists like Jennifer Lopez and Enrique Iglesias. The two began a romantic relationship. “I was in love,” Walton told the court, the Guardian reported. “She was my life then.”

They moved in together, and Walton financially supported Kouider as she tried to break into the fashion industry. As he told the court, in time Walton saw Kouider’s unhinged side. “She would go from softly spoken French accent, then she would flip, get very angry, very loud and just not care where we were,” he testified.

According to the Times, beginning in July 2012, Kouider began making wild claims to the police about Walton. She reported he was sleeping with male prostitutes. Walton also discovered Kouider had hidden cameras in the couple’s flat. After the pair split in 2014, Kouider’s bizarre reports to police continued — even after she returned to Medouni.

The Telegraph reported Kouider accused Walton of sexually abusing a cat, hiring helicopters to spy on her, and even targeting her with black magic. After creating a fake Facebook profile to falsely claim Walton was a pedophile, she was cautioned by police to stay away from Walton.

But for five years, her obsession continued, even while moving into the Wimbledon Park Road house with Medouni. Sunil Patel, the owner of a newsstand near the couple’s home, told the BBC Kouider brought Walton up at his store.

“She was obsessed with this guy,” Patel said. “She said, ‘Have you seen this person?’ and she showed me this photo of a blond-haired man on her phone.” Kouider told Patel the man was “a very dangerous person” who had hurt her family. “She said the nanny was complicit.”

Fifteen months before her death, Sophie Lionnet had come to work for Kouider and Medouni. A shy 21-year-old with pale green eyes, the French woman had hoped the time in London would sharpen her English. But the circumstances on Wimbledon Park Road were dark. Lionnet was forced to sleep in a bunk bed in a room with the couple’s two young children. She was regularly denied food and rarely left the house, the Times reported.

“Her body language showed there was something wrong,” the owner of a local restaurant where Lionnet occasionally ate alone told the BBC. “Once she said Sabrina beat her. . . . I asked why, and she said she had dropped the butter.”

In reality, Kouider had already fixed the au pair into her delusions involving Walton, delusions Medouni now also believed. At the trial, prosecutors stated that during the last six weeks of Lionnet’s life, the couple had become convinced Walton had seduced Lionnet, and that in return the au pair was drugging Medouni so Walton could sneak into the home and sexually abuse Medouni.

The couple began starving Lionnet and subjecting the au pair to brutal interrogation sessions, beating and dunking her head underwater until she admitted she was in league with Walton, according to testimony. The couple recorded the sessions on their cellphones. Police recovered eight hours of interrogation footage.

“So where is [Mark Walton’s] house? Because you fancy him, he is charming. . . . I don’t think he abused you, you wanted it,” Kouider told the au pair in one of the final sessions, according to the Telegraph. “And whenever you come back to the house, I smell sex.”

Police believe at one point in one of the interrogations in September, Lionnet was killed in the bathroom. According to the Telegraph, she was drowned, but her remains also showed she had fractures to her sternum, four broken ribs and a shattered jaw.

In court, Medouni and Kouider each blamed the other for taking Lionnet’s life. Kouider also told authorities Medouni forced her to have sex near the au pair’s dead body after the murder. Both, however, admitted to plotting to get rid of the remains in a bonfire in the garden. The couple cooked chicken out back in a flimsy attempt to mask the smell of the burning body.

In his court testimony, Walton confirmed he had never met Lionnet and only learned her name after hearing from police following the couple’s arrest.

The pair will be sentenced on June 26.

“Our Sophie will soon be laid to rest,” Catherine Devallonne, Lionnet’s mother, told the BBC. “No god will ever forgive you both for what you have done to our daughter.”