A British couple has been found guilty of murdering their French nanny in a bizarre case that a judge said was the result of a “folie a deux,” or shared psychosis.

The Guardian reports that Sabrina Kouider, 35, and Ouissem Medouni, 40, were found guilty this week of brutally torturing and murdering au pair Sophie Lionnet, 21, before burning her body in a bonfire.

The married couple was reportedly obsessed with British pop star Mark Walton, the founding member of Boyzone, who Kouider, a fashion designer with two children, has previously dated. According to testimony at the trial, Kouider had invented a dark fantasy that Lionnet was involved with and conspiring with Walton.

According to the BBC, Walton said at the murder trial that he had never met the victim.

Kouider and Medouni are believed to have starved and tortured Lionnet in the weeks leading up to her death in September, subjecting her to violent “interrogations.” It was following one of these interrogations, while Lionnet was in the bathtub, that she eventually died and the couple repeatedly pushed her head under water. Because her body was so severely burned, investigators have never determined the exact cause of her death.

Instead of calling emergency services, the couple burned Lionnet’s body in the garden of their south London home. When neighbors reported smoke, Kouider and Medouni reportedly said they were burning a sheep.

A firefighter who arrived to the scene saw signs of human remains, though Medouni reportedly tried to insist it was a sheep carcass.

Later, Kouider and Medouni reportedly admitted to disposing of Lionnet’s body, but each blamed the killing on the other. An unidentified witness in the home, who legally could not be named, reported seeing and hearing both of them in the bathroom with the Lionnet the night she died.

Ruth Bowskill, a chief prosecutor at the Crown Prosecution Service, told the BBC that Lionnet had written to her family in France saying that she wanted to leave the home, but may have felt trapped, particularly as she was very shy and did not speak English fluently.

“Given the intimidation, the bullying and the behaviour towards her… it’s likely that she didn’t feel able to leave,” Bowskill said.

At the murder trial, the victim’s aunt praised her kindness.

“She was a pearl. Kind, gentle. She loved everyone, she loved making people happy,” Lionnet’s aunt said.

The couple is expected to be sentenced in June.