The man suspected of murdering Wakefield schoolgirl Elsie Frost died following a heart attack, an inquest has been told.
West Yorkshire Police detectives, who had been re investigating Elsie's death since 2015, were preparing to bring charges against convicted child killer Peter Pickering, when he died on March 25.
An inquest at Reading Town Hall heard Pickering, 80, died in hospital in Berkshire, after suffering a heart attack at a secure psychiatric unit.
He had been locked up there for more than 45 years, after killing 14-year-old Shirley Bold in Barnsley in 1972, a crime which led to him. being nicknamed the Beast of Wombwell.
Pickering's unexpected death prompted police to say publicly for the first time that he was the man believed to have murdered Elsie.
The senior coroner for Berkshire, Peter Bedford, said: "The reports currently available show he was feeling unwell on March 25, complaining of back and stomach pain.
"He was reviewed by a doctor in the hospital, who asked for a urine sample, and he was left alone for no more than five minutes before a nurse returned to find Mr Pickering collapsed on the floor, clearly in pain.
"An ambulance was called, but he sadly went into a cardiac arrest.
"Despite CPR being administered he passed away in Thornford Park Hospital."
In the week before his death, Pickering was found guilty of the violent rape of an 18-year-old woman in Sheffield in 1972.
The crime came to light as a result of a renewed investigation into Elsie's murder, launched in 2015 on the 50th anniversary of her death.
Elsie was 14 when she was brutally attacked as she entered a railway tunnel on the afternoon of October 9, 1965.
She was making her way along a towpath next to the Calder and Hebble Canal in Wakefield after an afternoon with friends at Horbury Lagoon.
Elsie's brother Colin Frost said Pickering's left his family feeling "cheated" that they would never get justice for Elsie after a five decade search for answers.
He said: "In three words, I feel frustrated, disappointed and cheated... we aren't going to get the justice for Elsie that we really have worked hard for."
Pickering's inquest was adjourned until June 18.