Detectives investigating allegations of historical sexual and physical abuse on inmates at County Durham Young Offenders' Centre have begun interviewing former staff.
The alleged abuse took place between the 1960s and when the centre closed in 1988.
Since August last year, 915 former inmates have contacted police to report they were the victims of physical or sexual assaults.
The investigation into alleged historical sexual and physical abuse at Medomsley Detention Centre in County Durham comes a decade after an earlier investigation that led to the conviction and jailing of two former members of staff, Neville Husband and Leslie Johnson, who have since died.
Durham Constabulary said around one third of the 915 men who had contacted the latest investigation, which is named Operation Seabrook, reported being sexually assaulted by either Husband or Johnson.
The Operation Seabrook team has questioned four former members of staff who were prison officers at Medomsley during the 1970s and 1980s and have since retired, police confirmed.
All four were interviewed under caution, but not arrested, and could be interviewed again in future.
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