A notorious killer found guilty last week of raping a teenager weeks before he took the life of another has died.

Announcing the death of Peter Pickering, police said they had hoped to charge him with the murder of a third teenage girl.

Pickering, who was nicknamed the Beast of Wombwell after he admitted the manslaughter of 14-year-old Shirley Boldy in 1972, died on Saturday night after being taken ill in secure psychiatric accommodation in Berkshire, police said. He was 80 and his death is not being treated as suspicious.

Last week he was found guilty of the rape and false imprisonment of an 18-year-old woman in South Yorkshire three or four weeks before he killed Shirley.

On Sunday, West Yorkshire police said they “strongly suspected” Pickering was also responsible for the killing of Elsie Frost, 14, who was stabbed to death in a tunnel beneath a railway line in Wakefield in 1965.

Officers had interviewed him recently about the crime and expected he would soon be charged with her murder, the force said.

It was while reinvestigating Frost’s killing that police found a secure lockup facility in Sheffield that Pickering had kept since he was incarcerated in 1972.

Police believe he had somehow been paying rent on the storage unit over the decades, preserving exercise books, documents, letters and diaries, as well as a pair of handcuffs. The material suggested he had a long-held desire to commit violent offences against young girls.

One letter, written by Pickering to his mother, suggested he had raped an 18-year-old woman weeks before killing Boldy in 1972.

Detectives went to interview the victim, now a woman in her 60s. She told them she had thought she was going to be killed when she was violently raped by Pickering in his minivan near Stocksbridge, a town north of Sheffield on the edge of the Peak District.

West Yorkshire police said of the Elsie Frost case: “We strongly suspected that Peter Pickering was responsible for her murder. We had been liaising with the Crown Prosecution Service and it was our expectation that Pickering would be charged in due course.

“His unexpected death clearly means that will no longer happen. We have informed all those involved in the case of this development and we will be liaising with the West Yorkshire coroner as to what proceedings are now necessary.”

Interest in the case was reignited in 2015 when Radio 4 ran a series, Who Killed Elsie Frost?, prompted by a plea by Elsie’s older sister, Ann Cleave, for the crime to be solved.