A child killer dubbed the Beast of Wombwell died after suffering a heart attack at a secure psychiatric unit, an inquest has heard.
Peter Pickering, 80, had been locked up for more than 45 years after killing 14-year-old Shirley Boldy in the Barnsley area of South Yorkshire in 1972.
Announcing his death, West Yorkshire Police confirmed he was expected to be charged with the murder of another 14-year-old, Elsie Frost, whose body was found in Wakefield in 1965.
Her brother, Colin, said the family felt they had been "cheated" out of getting justice and believed Pickering could be responsible for other murders.
The sex attacker was also awaiting sentence for raping an 18-year-old woman, who is now in her 60s, a few weeks prior to Shirley's abduction.
An inquest at Reading Town Hall heard Pickering died at Thornford Park Hospital, in Berkshire, on March 25.
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 itself."
The coroner said a post-mortem exam found Pickering had died from a retroperitoneal haemorrhage due to a ruptured abdominal aortic aneurysm. It also showed he was suffering from heart disease.
Mr Bedford said: "Although that is a natural cause of death, because Mr Pickering is a detained patient I am obliged to have an inquest."
He adjourned the full inquest to June 18.
Pickering had been held under a hospital order made by a judge in 1972 after he admitted Shirley's manslaughter by diminished responsibility.
She was bundled into Pickering's van as she was returning to Wombwell High School.
Pickering drove her to a secluded location where he tied her up and raped her.
He tried to strangle her before stabbing her to death, a crime witnessed by walkers who were too far away to intervene.
Pickering killed Shirley just three or four weeks after he abducted and violently raped an 18-year-old woman in the Barnsley area.
Last month, the victim described to a jury how Pickering had told her she was going to die. She called him a "monster".
Pickering was convicted at Leeds Crown Court of rape and false imprisonment over the attack, which only came to light through the re-investigation of the murder of Elsie Frost.
Elsie was stabbed in the back and head as she walked through a railway tunnel in Wakefield, West Yorkshire, in October 1965.
As part of the inquiry detectives looked back through Pickering's conversations with psychiatrists and also found a storage garage he rented in the Owlerton area of Sheffield containing possessions including handcuffs and exercise books filled with his rantings.
One note written in 1970 said: "Sex is predominant in my mind- eclipsing all else. Maybe I will be a sex maniac proper. Rape, torture, kill."
Speaking after Pickering's death, Elsie's brother, who pushed for the re-investigation of the case three years ago along with his sister, Anne Cleave, said: "It's just an incredible feeling of frustration now."
He said the police had done a "fantastic job" but criticised the length of time prosecutors had taken to decide on a charge after detectives handed over the file to the Crown Prosecution Service a year ago.
Mr Frost said: "They uncovered a monster. The man was such a nasty, nasty piece of work."
Asked if he believed Pickering could have killed others, he said: "I think there's every likelihood."
Pickering had convictions for sex offences dating back to the very early 1960s and was in prison from 1966 to early 1972 for a violent sex attack on a teenager.