Police investigating sexual abuse at a Durham detention centre say they believe they have uncovered an organised paedophile ring operating in the 1970s and 80s with more than 500 potential victims.

The head of a 70-strong major inquiry into historical abuse at the Medomsley detention centre, near Consett, told the Guardian the inquiry was triggered by mounting evidence about isolated individuals. However, they were now investigating a complex paedophile ring, with many more victims than previously thought.

D Supt Paul Goundry, senior investigating officer, said: "We always knew this would be a major inquiry but the scale of it, and the sheer number of victims who have come forward, has been a shock."

He said that as well as sexual abuse they had evidence of a "brutal regime where violence was both extreme and routine". He suggested that prosecutions would be pursued for the offences, some of which occurred more than 40 years ago.

Nearly 100 men who had come forward were already receiving therapy via a local sexual assault referral centre and others had sought support from the children's charity NSPCC.

The inquiry began after the Guardian revealed in 2012 the plight of several victims of a prison officer at the centre, Neville Husband, suggesting that his abuse there, which had been known about for several years, was more extensive than previously thought.

Husband, by then a church minister, was jailed for eight years in 2003 for committing sex attacks on five young male Medomsley inmates between 1977 and 1984. His sentence was increased to 10 years in 2005 after new victims came forward and he admitted to attacks on four more boys.

Husband had been in sole charge of the kitchens at Medomsley. The 2012 Guardian investigation revealed he had raped boys on a daily basis for more than 15 years, while other staff allegedly turned a blind eye. His former colleague Leslie Johnson, a storeman, was jailed for six years in 2005 for sexual offences.

The pair, who have since died, were originally thought to be "bad apples", but now Durham police are convinced that Husband was part of a group of paedophiles who systematically preyed on vulnerable teenagers, particularly those who had been in care and did not have family who would visit them.

Medomsley Detention Centre, near Consett, County Durham Photograph: NNP

The investigation into abuse at Medomsley is the biggest undertaken by the police force.

Goundry said his officers had been shocked and sickened at evidence given by former inmates. "From the statements, there is growing evidence to suggest there was an organised paedophile ring operating in Medomsley. This will form a major part of our operation and future discussions with the CPS [Crown Prosecution Service]."

Goundry said the purpose of the investigation was threefold: to encourage victims to come forward, to prosecute surviving abusers and to help victims with belated after-care. He said officers were dealing with many men who were sent to Medomsley for petty crimes and had their lives ruined by the experience – some of them have never managed to work and still cannot leave their houses.

The investigation was also looking at victims of physical abuse at Medomsley. The team talked of a culture of violence above and beyond what was regarded as an acceptable level of discipline at the time.

In 2009, the then Labour government paid out a total of £512,000 in compensation to 12 Medomsley victims. For six years, the Home Office had fought compensation claims, using the statute of limitations to avoid payment in a costly legal fight that went all the way to the House of Lords, and even suggesting that one victim was "genetically predisposed" to being abused.

Those who were compensated hoped for an apology from the government, but the home secretary at the time, Jack Straw, told them "the terms of the agreement did not include an apology".

Kevin Young was sent to Medomsley in 1977 for receiving stolen property (a watch his brother had given him). In 2012, he told the Guardian he had been raped on a daily basis by Husband; sometimes at the centre, and other times at Husband's home by a number of abusers.

Kevin Young, who was abused at Medomsley detention centre when he was 17. Photograph: Gary Calton for the Guardian

After being released a day before his 18th birthday, Young went straight to Consett police station to report what had happened at Medomsley and was told it was a criminal offence to make such allegations against prison officers when he was on licence. "They were basically threatening to take me back to Medomsley, so I scattered pretty quick."

Young said this week he was not surprised by the scale of the current police operation, just saddened that it hadn't happened earlier. "In 2000, when I told police what I suffered and saw at Medomsley, I knew Husband was one of the most prolific sex abusers this country has seen. Despite all the alarm bells, he was allowed to abuse boys throughout his whole career as a prison officer."

• Anyone with information that may assist the inquiry can contact Durham police on 101 or 0345 60 60 365.

Timeline

1969: Prison guardian Neville Husband is arrested for possession of pornographic material while working in a kitchen at Portland Borstal in Dorset. The material depicts teenage boys. Police and the prison service accept his explanation that he was researching a book on homosexuality.

1970: Husband moves to Medomsley.

1977: Kevin Young is sent to Medomsley and immediately after his release reports being raped by Husband to local police. He is told he will be arrested for making complaints about prison officers.

1985: Storeman Leslie Johnson is arrested at Medomsley. Prison officers find pornographic material and sex aids in Husband's locker. Husband is transferred to Frankland prison. No action is taken against him.

1990: Husband retires from the prison service and receives an Imperial Service medal for serving with "diligence and fidelity".

1990: Johnson pleads guilty to sexually assaulting two boys, one at Medomsley. He receives a nine-month suspended sentence.

2003: Husband is jailed for eight years for attacks between 1977 and 1984.

2005: Husband's sentence is increased to 10 years after he admits further abuse.

2005: Johnson is jailed for six years for separate sexual offences.

2007: The CPS decides it is "not in the public interest" to proceed with charges that Husband abused a boy at Deerbolt youth offenders' institution after he left Medomsley and Frankland.

2009: The government pays £512,000 in compensation to 12 victims.

2010: Husband dies.

2012: The Guardian reveals that Husband's abuse was on a greater scale than previously thought. Police re-open the investigation after more victims come forward.

2014: Durham police say they are investigating an organised paedophile ring operating from Medomsley with more than 500 potential victims.