Notorious UVF leader Billy Wright was working as an agent for the police and military, senior security sources have told the BBC.

And according to the sixth episode of Spotlight on the Troubles: A Secret History, Wright's predecessor as leader of the Mid-Ulster UVF, the late Robin Jackson, had also been recruited as a state agent.

Jackson, who led the UVF terror campaign in Mid-Ulster from the 1970s through to the early 1990s, is thought to have been personally involved in up to 50 killings during the Troubles.

He had been arrested in 1973 for involvement in a murder, but was never prosecuted despite being identified by the wife of the victim. Charges were dropped, and a number of security sources have told the BBC they believed that was when he was recruited in return for avoiding prosecution.

Former Police Ombudsman Nuala O'Loan told the BBC: "My understanding would be that he was a murderer, a prolific murderer, a very, very dangerous and ruthless man. They never investigated him."

The BBC claims Jackson's murder gang included soldiers and RUC officers.

Billy McCauley, a former police officer and accomplice of Jackson in the murder of a Catholic shopkeeper, told the programme: "It would have been a case of meeting republican terror with even greater loyalist terror. That would have been the rationale."

The programme's research shows the number of attacks on Catholics - particularly on family members of those connected to republicanism - by the Mid-Ulster UVF rose dramatically when Wright took over as leader of the organisation.

Retired Detective Chief Inspector David Hoare, part of the Historical Inquiries Team, said evidence suggests the RUC didn't try hard enough to stop the UVF murder gang.

"Forty odd murders and so few people convicted - to me it tells a tale in itself," he said.

"It raises the question: did the RUC try hard enough or were they not good enough to deal with Mid-Ulster UVF?

"I don't buy the argument they weren't good enough.

"They were certainly good enough."

He also revealed a catalogue of missing evidence in cases relating to the Mid-Ulster gang when he went to reinvestigate the murders of Kevin and Jack McKearney in their butcher's shop in Moy in 1992, 13 years later.

The McKearney family had strong IRA links, though neither of the victims had involvement with paramilitarism.

"Crucial evidence had been lost," he said. "A partially destroyed jacket found in the getaway car had disappeared."

He also revealed in 1998 that hundreds of police files, including those on killings in Mid-Ulster, had been destroyed because of reported asbestos contamination.

"I can't say how huge the destruction of the records was," he said.

"There were health and safety measures that could have been taken to clean those exhibits safely, but that wasn't done."

Police told the BBC the evidential loss was minimal.

Belfast Telegraph