A child killer dubbed "the Beast of Wombwell" died of 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.
West Yorkshire Police said 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 "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 before Shirley's abduction.
An inquest at Reading Town Hall heard Pickering died at Thornford Park Hospital, in Berkshire, last month. The senior coroner for Berkshire, Peter Bedford, said Pickering had been found collapsed on the floor, in pain.
Pickering had been held under a hospital order made 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 raped her and tried to strangle her before stabbing her to death, a crime witnessed by walkers who were too far away to intervene.
Weeks earlier, he had abducted and 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.
Colin Frost 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 whether 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 sex attack.
PA
