More than 140 people have contacted police with allegations of physical and sexual abuse at the Medomsley Detention Centre, near Consett, during the late 1970s and early 1980s.
The centre first made national headlines in 2003 when Neville Husband, an officer at Medomsley, was sentenced to ten years in jail for systematically raping several teenagers.
One of Husband's victims, John McCabe, has claimed that he believes the abuse was covered-up.
Mr McCabe, who is now 48, has waived his right to anonymity, to speak of his horrific ordeal as a 17-year-old.
He said: “Within weeks of arriving at Medomsley (and the abuse starting) I was upgraded to the status of a most-trusted trainee, despite being a known flight risk. And Husband was the only person allowed to take me off the premises.”
Detective Superintendent Paul Goundry, who is leading the inquiry, codenamed Operation Seabrook, confirmed: “There are claims that there was cover-up that went right to the top and that Husband couldn’t have continued the abuse without others either turning a blind eye or helping.
After serving his sentence, Husband, a disgraced United Reformed Minister, died of natural causes at his home in nearby Snows Green, Shotley Bridge, in 2010.
His friend, storeman Leslie Johnson, was jailed for six year for similar offences. He too has died.
Police renewed their investigation after a meeting with Mr McCabe’s MP last August.
It has since become the largest inquiry of its kind, with about 80 officers involved, along with a support network – including Rape Crisis, The Meadows Sexual Assault Referral Centre and the NSPCC.
Det Supt Goundry said about half of the new victims alleged physical abuse “far beyond what was meant by a short, sharp shock intended”, while the remainder claimed sexual abuse - the bulk of whom said they had been been abused by Husband.
He added: “Every victim we have spoken to has a story to tell that is quite frankly horrendous – not only in terms of the details of the abuse, but the effect it has had on them since they were in Medomsley."
Det Supt Goundry said the investigation will move from evidence gathering to interviewing potential offenders by spring.
Former governor Tim Newell, who was in charge at Medomsley for part of the time the original abuse took place, has given a statement to BBC documentary Inside Out in which he says he was “very sad about the pain brought about by his staff”.
He adds: “If I had any suspicions about sexual abuse of any kind I would have taken action. If staff knew about the abuse taking place I am very concerned they let the abuse continue.”
Durham's chief constable, Mike Barton, pledged that all the allegations would be thoroughly investigated. "We are pleased so many people have come forward and trusted us with their accounts of what happened to them all that time ago," he said.
"It’s absolutely vital that all my staff who investigate allegations of sexual abuse treat victims sympthetically and provide them with the necessary support, and strive to bring offenders to book.
* Inside Out will be broadcast on BBC One at 7.30pm tonight.