They called him “Stakeknife”. Freddie Scappaticci, who died last year aged 77, was a Belfast bricklayer who spent more than a decade during the Troubles operating as a secret agent deep inside the IRA.

In March, the Police Service of Northern Ireland released its long-awaited report from Operation Kenova, a seven-year criminal investigation into the activities of the British Army agent, codenamed Stakeknife.

Scappaticci was not named in it directly, but as they say in Northern Ireland, the dogs in the street could tell you that Stakeknife was Freddie Scappaticci. As a spy, he passed on valuable intelligence to his British Army handlers and was described as a “priceless” asset.

He was also involved in serious crimes, including, it is alleged, as many as 14 IRA murders. Was the Army right to run Scappaticci as an agent for as long as they did?

The answer from Operation Kenova was damning and clear: “No”. There was not enough oversight of this agent and Scappaticci took more lives than he saved. Mistakes were certainly made and Scappaticci’s handlers did not have enough legal guidance.

But the Kenova report has raised almost as many questions as it has answered.

One of these was exactly why so many intelligence professionals told the Kenova detectives that Stakeknife was a valuable agent and that his intelligence had allowed them to save “countless” lives. Another question was how Scappaticci even came to be working for the Army in the first place.

There are all sorts of theories. Some have suggested he must have been blackmailed. Another rumour is that he walked into a police station and offered his services as a spy, or that a chance encounter in a pub with a British soldier set him on this path.

The truth lies elsewhere. I have spent the last five years researching and writing a book about one of the murders that Stakeknife is alleged to have carried out. Along the way, I’ve been able to piece together the details of how this agent was taken on.

The story starts one day in 1977, when a young staff sergeant in the British Army went to a police station in central Belfast to see a friend.

The soldier’s name was Peter and he was on his fourth emergency tour of Northern Ireland with the Devonshire and Dorset regiment, known in the army as the D’n’Ds. Peter’s friends would tell you that he was not your typical soldier.

He always seemed to have a twinkle in his eye, he spoke in a warm West Country burr and was someone who could charm the birds out of the trees. He was also more at home in Northern Ireland than most soldiers. He had recently married a local woman and was said to be a drinking buddy with local singer-songwriter George Ivan Morrison – better known by his stage name Van Morrison.

The reason Peter went to the police station that day was because a Special Branch detective had asked if he’d like to meet one of his informants.

This was almost unheard of. The police did not, generally speaking, like to share their agents with the Army. But this detective enjoyed spending time with Peter and therefore made an exception.

The informant he wanted him to meet that day was a well-built, fast-talking former IRA man called Freddie Scappaticci.

If Peter had assumed that Scappaticci was passing on details of what the local IRA brigade was up to, he was wrong. “Scap”, as he was known, was only supplying the police with names of builders who might be involved in a VAT tax scam.

He wasn’t doing it out of some sense of civic propriety, but because he had been caught taking part in the same tax fiddle himself. He was given a choice of going to jail or working for the police.

We’ll never know the precise details of what happened next – after Peter walked into the room and began to talk to Freddie Scappaticci – but we know that the two men got on. It would be wrong to say the plan was working, because there was no plan.

But a friendship had begun, and soon Peter began to wonder if one day Scappaticci might agree to work for him as an agent.

In the weeks that followed, Peter found ways to bump into Scappaticci again. On one of these encounters, he suggested casually that they go for a drink. Scappaticci said yes, and Peter went to meet him.

This was brave. At about the same time, Robert Nairac, a captain in the Grenadier Guards and a liaison officer attached to the SAS, went to a pub in South Armagh and spent the night singing rebel songs and speaking in a fake Northern Irish accent.

He was probably hoping to recruit an agent, but we’ll never know, as later that night he was abducted by the IRA, interrogated and killed.

“Nairac was the bravest man I ever met,” said one of the IRA volunteers later convicted of his murder. “He told us nothing.”

Peter knew that going for a drink with Scappaticci so soon after Nairac’s murder was a risk. Scappaticci could have told his former IRA comrades what was happening. This might be a trap. But Peter took precautions.

He made sure to meet in a pub in neutral territory. He also had a plan for what to do if it all went wrong. In the conversation that followed, the British soldier did not ask the former IRA man to work for him as an agent. Instead, he kept the conversation light. They chatted about music, women and football.

“The relationship between agent and handler is a marriage,” Peter later told a senior army officer, “but, a one-sided one.

“The handler must know everything about his agent: his fears, his personal problems, his concerns about money, how often he has sex and with whom, his relationship with his wife and who he hates within the IRA.”

The last point was vital. The key to understanding Freddie Scappaticci was that he had recently been kicked out of the IRA and was furious about what had happened. Peter recognised in Scappaticci a pathological need for revenge.

This was his way in. What better way to get back at the IRA than rejoining it and working against it from the inside? Eventually, he spelled this out to Scappaticci, who agreed to work for him. This recruitment had nothing to do with blackmail or a chance encounter in a pub. It was rooted in Scappaticci’s desire for revenge and the friendship that had formed between him and Peter.

In the months after Scappaticci first agreed to work for the Army, he proved himself to be a productive agent, passing on “tactical” details of imminent IRA attacks as well as “strategic” information about what was going on in the upper echelons of the organisation. Only after that did Scappaticci agree to rejoin the IRA.

This was dangerous, as the IRA had recently set up a new unit dedicated to hunting down spies called the “Nutting Squad”. To “nut” someone in Northern Ireland is to shoot them in the head. Scappaticci could not have known he would be allowed back into the IRA and later asked to join the same Nutting Squad, eventually rising to become deputy commander of this terrifying unit.

Several years later, in May 1986, another army agent from Derry, called Frank Hegarty, was abducted by the IRA and questioned by three senior paramilitaries. One was Martin McGuinness, who would later become deputy first minister of Northern Ireland. The next day, Hegarty’s corpse was found by the side of a road.

According to Scappaticci, McGuinness personally gave the order for Frank Hegarty to be killed. But who pulled the trigger? Usually a job like this would be handed to the Nutting Squad. Was it possible that one army agent – Scappaticci – had killed another? More than 10 years later, in 1999, a British soldier who had known Hegarty and remained upset by the news of his murder came to the conclusion that Scappaticci was responsible.

He went to the media and worked with a journalist on a story that revealed the existence of an army agent codenamed “Steakknife” (the spelling changed in later reports). As more details emerged, so did the pressure for some kind of investigation, which led, ultimately, to the launch of Operation Kenova in 2016.

One big question remains. Was the Kenova report that came out earlier this month correct to say that Scappaticci took more lives than his intelligence saved? The author of this report, Jon Boutcher, now Chief Constable of the Police Service of Northern Ireland, did not reveal exactly how he arrived at his conclusion. Perhaps more details will emerge later this year.

But Boutcher’s estimate of how many lives Scappaticci saved appears to have been a conservative one. He was only looking at lives that were saved as a direct result of a tip-off from Scappaticci. He did not consider the impact of the information he supplied that corroborated other reports, or was passed on to his handlers without a record being kept.

Nor did he take into account the lives that were saved each time Scappaticci’s intelligence led to a bomb being defused or an IRA attack being called off, as well as the broader impact of his intelligence, which helped to bring parts of this paramilitary organisation to a standstill in the years leading up to the Good Friday Agreement.

Boutcher was right to conclude that there was not enough oversight, and that Scappaticci’s handlers turned a blind eye to what their agent was doing on too many occasions.

But the report that came out earlier this month is only the start of this story and more is likely to emerge in the months to come.

Four Shots in the Night, by Henry Hemming, (Quercus, £22) is out now. Visit expressbookshop.com or call Express Bookshop on 020 3176 3832. Free UK P&P on orders over £25