It is a murder which continues to attract claim and counter claim even 40 years later.
What is not disputed is that Captain Nairac was kidnapped by the IRA after spending the evening of May 14, 1977, undercover at a Republican bar just inside the Northern Ireland border.
He was then bundled into a vehicle in the pub’s car-park just before midnight and taken into the remote Republic of Ireland countryside where he was tortured and eventually killed the
While his pistol and traces of his hair and blood were later found to support the events so far, the 28-year-old Grenadier Guardsman’s remains have still to be recovered.
Now comes the guesswork.
Why was Sunderland-raised Captain Nairac in the pub on his own in the first place?
Was he meeting a contact so that he could feed intelligence back to the British services at the height of the Ulster Troubles?
Was he - as the IRA quickly claimed - a member of the feared SAS regiment and therefore a notable scalp in the terrorist organisation’s battle for a united Ireland?
Did he also work with Loyalist or Protestant paramilitaries to orchestrate atrocities committed against Catholics in the supposed name of keeping Ulster part of the UK?
Or was he a loose cannon acting outside his limited authority as an army intelligence officer in posing as a native and singing pro-Republican songs just minutes before displaying his boxing prowess as his kidnappers struggled to overpower him?
The focus of the current investigation by the Independent Commission for the Location of Victims’ Remains is not to answer questions about Captain Nairac’s past.
It is primarily to find out where his remains and those of two other casualties of Republican terrorists are buried.
Even then the conjecture inevitably continues.
With the six people convicted in connection with his murder refusing to discuss the whereabouts of his body, the IRA drip-fed the theory that it had been fed to a meat-mincing machine.
The authorities, however, have gradually disputed this as a rouse to hinder the hunt for suspects who have to this day escaped arrest.
They also insist the IRA trumped up Nairac’s significance by linking him to the SAS and atrocities carried out by Protestant terrorists.
Relative silence at the time from the authorities, worried that confirming too much information would hinder future operations, inadvertently strengthened the Republican stories.
Commission lead investigator Mr Knupfer said: “I think it is an accumulation of unfortunate facts or circumstances that we are in this situation.
“There is an assumption that he was in the SAS and he wasn’t. There was an assumption that the right thing to say from the military side was no comment. There was an assumption that other people were looking after him when they weren’t.”
With Captain Nairac’s parents, Maurice and Barbara, who moved to Gloucester from Sunderland around 1970, now dead, it is his surviving older sisters, Rosamonde and Gabrielle, who are now seeking closure.