Two decades ago, a 41-year-old patient in a psychiatric hospital made a shocking confession. Sture Bergwall stunned his therapist by admitting he was responsible for one of Sweden’s most notorious unsolved murders, that of 11-year-old Johan Asplund, who had vanished on his way to school in 1980 and whose body had never been found.

Police were called to interview Bergwall about the case, but that wasn’t the end of his disturbing mea culpa. Over the next few years, speaking as “Thomas Quick”, a malicious alter ego, he confessed to a string of other unsolved murders.

His victims, he claimed, included a 15-year-old boy, a Dutch couple on a camping holiday, a nine-year-old Norwegian girl and a 23-year-old prostitute. He had variously tortured, raped and dismembered them, before eating their body parts.

Police dusted down case files of unsolved murders in the hope – often fulfilled – that Bergwall might lay claim to them. By 2001, he had confessed to 30 murders, had been convicted of eight, and had gone down in history as Sweden’s “Hannibal Lecter”, by far the worst serial killer the country had (at that point) known.

“He appeared in the news, eyes staring out,” says Swedish journalist Jenny Küttim, who has known Bergwall since 2008. “Everyone was aware of him and, even though he had confessed to many murders, thought he must have done many more.”

The Confessions of Thomas Quick, a documentary film airing tonight on Channel 4, tells of how Bergwall explained his crimes. He had suffered, he said, a childhood at the hands of depraved parents; sexual abuse had turned him into a monster.

It was at this point that the story took an extraordinary turn. In 2008, journalist Hannes Råstam decided to re-examine the 50,000 pages of therapy notes, court documents and police interrogations relating to Bergwall’s convictions. In tandem with Küttim, then his assistant, Råstam came to the conclusion that there was no evidence for any of them aside from Bergwall’s own confessions – many of which had been made when he was taking high doses of the mind-altering tranquilliser benzodiazepine.

Råstam visited Bergwall at Säter, the psychiatric prison where he had been incarcerated since 1991 for an armed robbery, and confronted him with this research. To his surprise, Bergwall broke down, and admitted that all his confessions were false.