Peter Pickering, 80, was last week convicted of raping an 18-year-old woman before he raped and killed a 14-year-old child in 1972, for which he had already been convicted.
He died on Saturday evening after falling ill at the Thornford Park Hospital secure psychiatric accommodation in Berkshire, according to police.
Police added that Pickering was also a suspect in a historical investigation into the 1965 murder of 14-year-old Wakefield schoolgirl Elsie Frost.
“We strongly suspected that Peter Pickering was responsible for her murder,” said DS Nick Wallen of West Yorkshire Police.
“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 in relation to Elsie Frost’s death.”
Shirley Boldy was 14 when Pickering abducted, raped and murdered her in Wombwell near Barnsley.
A woman in her 60s told a jury how she was also attacked by Pickering shortly before Shirley Boldy’s death.
The woman told the jury how Pickering lured her to his van before bringing her to a secluded spot and removed her underwear with a knife, handcuffing her and burning her breasts with cigarettes before raping her and telling her he was going to kill her.
She said: “I remain of the fervent belief that whether he has a mental illness or not the man is a monster and wherever he is now that’s the right place for him to be.”
Pickering was subjected to a hospital order without time limit in 1972 for the manslaughter of Shirley Boldy by reason of diminished responsibility, and had been held in facilities including Broadmoor since that date.