A post-mortem examination is being carried out on the body of child murderer Colin Hatch who was killed at one of Britain's most secure prisons.
Hatch, 38, of north London, died at Full Sutton jail near York on Tuesday.
He 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.
Inmate Damien Fowkes, 35, who is serving a life sentence for robbery, is being held on suspicion of murder.
Fowkes was told he must serve a minimum of 12 years by Northampton Crown Court in 2002 for carrying out three robberies over four days.
He was transferred to the East Yorkshire maximum-security prison last year.
The arrested man is in custody at the prison and will be moved to a police station at a later date, Humberside Police said.
The force said officers were called to the jail shortly before 1925 GMT after reports that "an incident was ongoing".
A short while later police received a call stating a man had died.
Hatch had a string of convictions from the age of 15 for assaulting young boys when he was jailed for the murder of seven-year-old Sean Williams in January 1994.
Jailing Hatch, 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, 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 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.
Hatch's murder conviction prompted a review of parole and probation for those convicted of sex crimes.
- 23 February 2011