The man associated with being one of the British Armed force's top agents inside the IRA admitted in 1990 that he had shot dead a suspected informer.

The detail has been uncovered in a court record during a BBC Spotlight examination concerning his exercises.

Freddie Scappaticci, who passed on in April, had consistently rejected that he was the specialist given the codename Stakeknife.

Stakeknife is thought to have been linked to more than 20 murders during the Troubles in Northern Ireland.

By 1990 Freddie Scappaticci had turned into the IRA's chief spy catcher within its internal security unit.

It was known as the "nutting squad" because the informers it uncovered were shot in the head - the nut - and their bodies were dumped after interrogations involving torture.

In February 1989 Joe Fenton was shot dead after being interrogated by the IRA's internal security unit in a house in west Belfast.

No one has ever been charged with his murder.

Several months later, another suspected informer Sandy Lynch was brought to the same property to be interrogated by Freddie Scappaticci and others.

Spotlight has spoken to Gerard Hodgins, a former IRA prisoner, who was also one of the individuals in the house where Sandy Lynch was being held.

"I had no idea that his name was Sandy Lynch at this time - we were just told to go to a house and babysit, the guy was an informer," he told the program.

However, unlike many suspected informers within the IRA's ranks, Sandy Lynch survived after the police raided the property.

By then Freddie Scappaticci had already left the house but he had left something behind - fingerprints on the battery of an anti-bugging device.

A police fingerprint expert said he was certain that the imprint was made by "Frederick Scappaticci".

To avoid arrest Freddie Scappaticci went on the run to the Republic of Ireland for a time.

Sandy Lynch, who was a police informer, went on to give evidence in court before he entered a protection program.

As part of his court deposition, he detailed the IRA interrogation he was subjected to, including how he was stripped, blindfolded, tied up, and taunted for hours by his interrogators.

"One of them told me that he enjoyed his work and that he would break me," he said.

In his deposition, Sandy Lynch also made detailed references to Freddie Scappaticci.

"He said that I would wake up hung upside down in a cowshed and he would talk to me the way that he wanted to talk to me, that he would skin me alive, and that no one would hear me squealing."

The body of Joe Fenton had been left in an alleyway just meters from the house where Sandy Lynch was being interrogated.

Sandy Lynch's deposition also included graphic remarks that he said Freddie Scappaticci had made about the manner of Joe Fenton's death.

He said: "[Freddie Scappaticci] tapped me two or three times on the back of my head and said: 'You'll get it right there... like that b*st*rd Fenton.'

"He said that he had done it."

But it seems that Freddie Scappaticci was untouchable because by the time he was arrested, it was in connection with his fingerprint and he had already concocted a false alibi.

Gerard Hodgins is one of eight people whose convictions in relation to the Sandy Lynch episode were later quashed after it emerged that crucial information had been withheld from prosecuting authorities.