Moors murderers Myra Hindley and Ian Brady tortured and murdered five children in and around Manchester between 1963 and 1965.
But Brady remains were disposed of in Merseyside, somewhere off the coast.
Channel 4's Moors Murders documentary, which aired this week and is available online, told the story of the killer pair, whose murder spree only ended when Hindley's brother-in-law, David Smith, reported them to the police after witnessing their final murder.
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The three-part series shows never-previously-seen prison letters between the killer couple.
Pauline Reade, 17, John Kilbride, 12, Lesley Ann Downey, 10, and Edward Evans, 17, some of whom the murderers sexually assaulted, were buried on Saddleworth Moor in the south Pennines.
Both Brady and Hindley died without revealing where they buried the body of 12-year-old Keith Bennett, who vanished in 1964.
Myra Hindley died in hospital from bronchial pneumonia in 2002, after several failed appeals against her whole life sentence.
Her death led to a lengthy hunt for a funeral director willing to make her funeral arrangements.
Ian Brady was diagnosed as a psychopath in 1985 and moved to the high-security Ashworth Hospital in Maghull, where he died of natural causes in 2017.
The killer was cremated without ceremony in a secret operation at Southport Crematorium.
His ashes were then placed in a weighted urn, driven to Liverpool Marina, and dispatched at sea around 2.30am on Thursday, October 26, 2017.
At the time, Sefton Council's then-chief executive Margaret Carney said: " The High Court ordered us to cremate the remains of Ian Brady because he died within the Sefton borough boundary.
"In complying with this Order from the High Court, the cremation took place at Southport Crematorium outside normal operating hours and no other services at the crematorium were affected.
"The coffin did not enter any public area and was cremated in a separate standby cremator which was professionally cleaned afterwards.
"Following cremation, the ashes were immediately returned to the administrator of the estate and not disposed of anywhere in the borough."
