By the mid-1980s the RUC, Garda and British Army were accustomed to picking up the bodies of 'informers' shot dead by the IRA, usually in ditches or fields in isolated rural areas.
The Derry city IRA murdered around a dozen of its own members and civilians, Protestant and Catholic, for informing or simply helping the police with witness statements to IRA crimes.
One of these victims stands out in police officers' memories of the time. Frank Hegarty was an IRA man who is believed to have been acting as one of several informants inside Martin McGuinness's Derry 'Brigade'. Hegarty had been an agent for several years. He was possibly 'turned' after being arrested for the double murder of two civilian cleaners, a man and woman in their fifties, who worked in the British Army's main barracks in Derry city in the mid-1970s.
In early 1986 Hegarty was aged 45 and by then suffering from alcohol addiction and stress after leading this highly dangerous double life for more than a decade. He felt on the verge of being exposed and killed by the IRA. He fled the city and with the help of his British Army handlers was relocated to a house in Kent, near an Army barracks.
He did not, however, settle down and felt homesick. His family sought and received assurances from Sinn Fein in Derry that if Hegarty returned and confessed his role as an informer his life would be spared. McGuinness gave personal assurances, Hegarty's mother later said.
Hegarty duly returned, made a public statement at a Sinn Fein press conference, then disappeared. His body was found lying face down in a small stream running alongside a quiet country road near the Border at Castlederg, Co Tyrone.
The double-cross was not what surprised police officers, or anyone. Hegarty had a low IQ and was evidently deranged if he felt a simple confession would save his life. The officers who recovered his body were perplexed that before he was shot his executioners wound an entire roll of electrical insulating tape around his head covering the eyes and nose but allowing him to breathe for the short time left before being shot twice in the head.
The police hadn't seen this type of preparation before. What they hadn't know but learnt later was that after the previous execution of an informer in Derry there had been a scandal in the Republican community over claims that the victim had been tortured and his eyes gouged out.
The previous victim was Kevin Patrick Coyle (24) and his family contained many prominent Republicans, though this hadn't saved his life. He was found shot dead on the outskirts of the city in February 1985. He had been shot in the head twice, as normal, and had fallen face-first into a stream. His killers hadn't noticed that the handgun and new, more powerful ammunition they had used had caused such an explosive force inside Coyle's head that the eyes had been blown out of their sockets. The eyes and optical nerves had fallen into the stream and floated away. The IRA had a habit of leaving booby trap bombs around the corpses of informers, so it usually took a day or two to clear the area and recover the remains. The eyes could not be found.
The ensuing scandal was difficult for McGuinness and Sinn Fein in the city and it was clear this wasn't to reoccur. So, when Hegarty was taken away, the insulating tape was brought along to ensure that his eyes stayed in his head when he was shot.
The Derry IRA's campaign consisted largely of murdering off-duty members of the police, Ulster Defence Regiment and, less and less frequently during the 1970s and 1980s, British soldiers. The Derry IRA took the lead in Sinn Fein's campaign against all things to do with the British civil administration of Northern Ireland, including the taking of the 1981 census. They shot dead one of the census collectors, Joanne Mathers, a 29-year-old married woman with a two-year-old son in April 1981 as she collected forms from an estate in the Gobnascale area of the city.
Despite its routine, low-level murder campaign, the Derry IRA, under McGuinness's direction, is remembered for one particularly atrocious, if highly successful operation, the 'proxy/suicide' attack on the British Army Border checkpoint on the Buncrana Road on the outskirts of the city in October, 1990.
Patsy Gillespie, 42, a married man and civilian worker in the Army's main Ebrington Barracks, was taken prisoner at home. He was told he was to drive a van bomb to the Army checkpoint and that a timer on the bomb would allow him to raise the alarm and escape before the explosion.
He was told if he didn't comply his family would be murdered. A clock with wires attached was placed on the passenger seat beside him and it indicated that he had plenty of time for the short journey and sufficient time to escape before the explosion.
The clock was a ruse. The IRA had wired the bomb to the vanity light switch in the van door. When Gillespie arrived at the checkpoint, he opened the door to raise the alarm and detonated the bomb, killing himself and five soldiers.
This event is still studied on police and military counter-terrorism courses. It was adapted for use by terror groups in the Middle East and elsewhere later as the IRA shared and sold its technology and expertise.
The Gillespie proxy bombing was the Derry IRA's swansong and the RUC and Garda clamped down hard. Another proxy bomb with the same fake clock was captured by the Garda in Donegal just as it was being prepared.
The 'officer commanding' of the Derry IRA, the man directly under McGuinness, was recruited by the RUC Special Branch and became one of its most important agents. He was so successful that on one occasion the Branch officers hired suites in the famous Gleneagles Hotel and golfing resort. While the policemen played golf, prostitutes were provided for a weekend's entertainment with their agent.
This man disappeared after he was arrested by gardai in Donegal and, facing charges of attempting to rob an off-licence while he was drunk, decided to spill the beans and finally gave up his role and opted to leave Ireland. He has not been heard of since but is believed to be still alive.
The level of killings by the IRA in Derry fell to almost zero in the last four or five years of the Troubles. Belfast was similarly heavily infiltrated with dozens of informers up to and including commanding officers working as informants.
As overall military commander of the Provisional IRA, McGuinness is still held in high regard by counter-terrorism people around the world and his work continues to be studied. His ascension to the top of the IRA by his early twenties reflected not only his technical and tactical expertise but also his ability to instil fear and respect in a large and diverse terrorist organisation.
By the time of the Gillespie attack the IRA was already beaten and almost the entire organisation was infiltrated with informers right up to its seven-man 'Army Council'.
The belated switch to politics allowed McGuinness and Gerry Adams the path out of terrorism. While Adams made the arguments to convince the Provos of their new path, it was McGuinness's evil reputation that led the 'Army' to follow orders and give up the bomb.