Police are set to question up to 100 ex-staff at a youth detention centre after 900 former inmates claimed they were abused.
Police are questioning former warders from a youth detention centre at the heart of Britain's biggest sex abuse investigation.
Up to 100 former staff at Medomsley Detention Centre could eventually be questioned as part of the probe, it is thought.
More than 900 men have made accusations of rape, other sex attacks and violence during their time at the Medomsley Detention Centre near Consett, County Durham, in the 1960s, 70s and 80s.
Det Supt Paul Goundry said: "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."
The suspects are being questioned under caution after volunteering to meet police.
No arrests have been made so far.
Eric Sampson says he was abused at Medomsley at the age of 17 by a prison officer who threatened to kill him if he spoke out.
He told Sky News: "It was worse than a concentration camp. It was ran on violence, Medomsley, it was a hellhole.
"It was the most shocking, horrible place I've ever been in my life."
The historical investigation was launched 15 months ago and 70 police officers are now involved.
It is not the first time the now-closed detention centre has been investigated over allegations of abuse.
In 2003 prison officer Neville Husband, also a church minister, was jailed for sex attacks on five young males.
His sentence was later increased after other victims came forward.
His colleague Leslie Johnson was jailed for similar offences in 2005. Both men are now dead.
At the time they were considered "bad apples", but since then hundreds more former inmates have made allegations before the launch of Operation Seabrook, now thought to the the UK's biggest such investigation.
Michael McCann MP, who was instrumental in prompting the new investigation, said: "The questioning of so many suspects demonstrates that the brave victims are being taken seriously.
"The scale of the abuse is absolutely shocking."
Some victims claim they were taken outside the centre to be abused, suggesting the suspected paedophile ring included other men not connected to Medomsley.
Medomsley abuse: Durham Police interview former wardens
13 November 2014 Last updated at 17:36
One former inmate told the BBC's Danny Savage he would have killed himself if he could because of the abuse at the youth detention centre
Former wardens at a youth detention centre have been interviewed by police investigating claims a paedophile ring operated there.
Allegations of abuse at Medomsley Detention Centre near Consett are being investigated by Durham Police.
More than 900 former inmates have come forward to say they were sexually or physically abused in the 1970s and 80s.
The detective leading the operation said the force would be contacting "a number" of other former employees.
"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," Det Supt Paul Goundry said.
The four prison officers, who worked at Medomsley at different times between the 1970s and 1980s, have been formally but voluntarily interviewed by detectives. They have not been arrested.
Danny Savage, North of England correspondent
When a grown man tells you he would have killed himself if he could, during his time at Medomsley, you know that any legal case won't make a huge difference to many of their daily lives.
Things went badly wrong behind the high security fences on a County Durham hilltop decades ago.
Unspeakable things were done to some of those young men - often there for things you would never be jailed for today.
After locking away those painful memories for so long they are now speaking out, hoping for justice.
The investigation was triggered after former prison officer Neville Husband was jailed for eight years in 2003 for abusing five youths.
The publicity surrounding the trial led to others coming forward and he was subsequently jailed for a further two years for other attacks.
He died in 2010, after being released from prison.
His former colleague Leslie Johnson, who was jailed for six years in 2005 for sexual offences, has also since died.
Police no longer believe the pair were operating alone.
One alleged victim, Stephen Branley, described how the experience had made him suicidal.
"They would punch you in the ribs and in the head, bending your fingers back. I wanted to kill myself," he said.
"If I'd had the chance to get hold of a razor blade I would've done it."
Another alleged victim, who spoke anonymously, previously told the BBC: "Some of the boys would lay at the bottom of the stairs and ask another boy to jump off the stairs on to their legs so they could break a leg and be removed from Medomsley Detention Centre in order to not be subjected to any more beatings."
Another, Ray Poar, has waived his right to anonymity. He was 17 when he was sent there for stealing biscuits from a factory and said the experience "ruined my life".
He said he was woken up after wetting his bed and was forced to bunny hop naked to the showers.
When he couldn't make it to the showers he was kicked, he said.