November 13, 2014 17:00 GMT
The investigation into Medomsley Detention Centre, near Consett - named Operation Seabrook - was launched last August following allegations by former inmates at the detention centre from the 1960s to when Medomsley was closed in 1988.
Previous police investigations in 2003 and 2005 led to the conviction of former Medomsley staff workers Neville Husband and Leslie Johnson, both of who have since died.
Since Operation Seabrook was launched, a total of 915 former inmates at the centre have come forward with allegations of sexual or physical abuse, one-third of whom say they were sexually assaulted by either Husband or Johnson.
Four men have now been questioned, but not arrested, by police after voluntarily attending police stations in either Durham or Consett.
All four men were prison officers at Medomsley at different times during the 1970s and 1980s and have since retired. They were free to leave, but warned they could be questioned again in the future.
Det Supt Paul Goundry, of Durham Constabulary, praised the "courage" of the alleged victims in coming forward and said nearly 200 out of 915 of them had taken up the force's offer of counselling and professional support.
He added: "Many of those who have contacted us had never revealed to anyone else what had happened to them at Medomsley all those years ago. It has been a traumatic experience for some, and I appreciate their courage in coming forward and making that initial call.
"A principal aim was to identify a number of people we needed to speak to about the allegations that have been made. The actions we are now taking are crucial to that aspect of the operation, and there are a number of other former employees we will be making contact with over the next few weeks.
"We have been liaising with the Prison Officers Association over the last few months and anyone we interview is made aware of the legal support the POA can provide.
"There is still a huge amount of work which has to be done and we are in close contact with the Crown Prosecution Service, who ultimately will decide if there are grounds to charge individuals with criminal offences."