Police are investigating the suspected murder of child killer Colin Hatch at one of Britain's most secure prisons.
Hatch, 38, of north London, died at the maximum-security Full Sutton jail near York on Tuesday, police said. Post-mortem tests will be carried out later.
Hatch was jailed for life in 1994 after being convicted of killing a seven-year-old boy while on parole for a previous child sex attack.
A 35-year-old prisoner has been arrested on suspicion of murder.
He is in custody at the East Yorkshire prison and will be moved to a police station at a later date, Humberside Police said.
A force spokesman briefly described events at the jail on Tuesday night.
He said: "We received a call from the prison service at 1924 GMT reporting that there was an incident ongoing at the prison.
"At 2007 GMT we received a further call stating that a 38-year-old man who had been involved in the incident had died."
Jailing Hatch, who had a string of convictions for assaulting young boys, Judge Nina Lowry said he was "highly dangerous" and told him it was not possible to envisage a time when he could be released safely.
Just over two years before he was jailed for life in 1994, he had been jailed for three years for assaulting a boy of eight in almost identical circumstances.
His lawyer warned he could kill when he was released.
Within 11 weeks of being paroled in April 1993, Hatch struck again.
Sean Williams was abducted, sexually assaulted and then choked to death after Hatch lured him to his tower block home in Norfolk Close, Finchley, north London.
A postman discovered the youngster's body taped up in bin liners and dumped in a lift.
Det Supt Duncan Macrae, who led the murder inquiry, described Hatch at the time as a frighteningly cunning criminal.
'Menace to public'
He had pulled the wool over the eyes of the authorities and would kill again if he was ever released, Mr Macrae said.
Hatch had a string of previous convictions from the age of 15 for attacks on six young boys.
At his trial for indecently assaulting the eight year-old boy and choking him until he lost consciousness, psychiatrist Dr Anthony Wilkins recommended that Hatch was a "menace to the public" and should be sent to Broadmoor top security hospital.
But Broadmoor had not considered Hatch dangerous enough at the time, Dr Wilkins said. Instead he was jailed for three years, as the judge was given no alternative.
His murder conviction prompted a review of parole and probation for those convicted of sex crimes, with Sean's mother and father, Lynn and John Williams, saying others had to take some of the blame for their son's death.
Mrs Williams said: "Never again must a child be murdered by a pervert.
"Never again must a family have to suffer this experience and never again must Colin Hatch be released back into our community."