THE extent of Cullen’s abuse came to light thanks to a breakthrough thousands of miles away in Iowa, in the United States.

Officers had arrested part-time police officer Donald Rager for sex offences against children and possession of indecent images in 2007.

The then 39-year-old had a photo of Cullen raping a young boy, which officers seized.

The pair had met on the Internet with Rager purporting to be an 11-year-old called Billy.

Thames Valley Police’s Dc David Porter travelled to Iowa in January 2008 to interview Rager, later jailed for 31 years.

Dc Porter said: “Before the Rager case we suspected something had happened, but had no way of proving it. It was a breakthrough.”

He said Rager told how Cullen described sexually abusing several young children, including the victim. He also identified victims in photos that had been taken by Cullen and sent to him via the Internet.

Officers also went to Newcastle, where Cullen had befriended one set of parents via the internet, a Blackpool hotel where he took his victims, and Stafford.

Dc Porter said Cullen’s victims trusted him to the point where they initially refused to disclose what had happened to them.

He remembered how one victim, a boy from Oxford, eventually broke down in tears and admitted Cullen had raped him, another breakthrough in the case.

He said: “I think he realised we were trying to help. He saw there was a way we could stop this happening. But he’d become indoctrinated to it. To him, it was the norm.”

Cullen never appeared at court for sentencing, apparently feigning ‘illness’, according to police.

Dc Porter added: “I hope they don’t let him out for a long, long time. If he comes out he will probably carry on in the same vein. He never wasted an opportunity to abuse children.

“The sentence was a relief for the children, as some of them had been keeping it locked up for years. Oxfordshire is now a safer place.

“But the main thing is how brave the children were. They led the prosecution against Cullen and their bravery needs to be recognised.”

Earlier this month, Dc Porter was commended by Chief Constable Sara Thornton for his investigation.

He said: “You never give up and you try and look under every stone.”

Helping Det Con Porter were public protection officer David Eustice, Pc Tracey McClean, and Dc Sarah Wood.