An agent for police in Northern Ireland was killed by the IRA despite another British spy warning his handlers he would die, reports said.
Joe Fenton was secretly providing information to the Royal Ulster Constabulary's special branch and was suspected by the armed group of being an informer.
Bedfordshire Police Chief Constable Jon Boutcher is investigating the high-ranking Army mole, Stakeknife, who led the republican organisation's "nutting squad" internal security unit which interrogated and murdered suspected spies.
His team is probing more than 50 murders and the BBC's Panorama links Stakeknife to at least 18.
The chief constable told the programme: "We need to understand what was the rationale and decision-making of one person being allowed to die in order potentially, if this was the case, that another person can live."
Father-of-four Fenton, 35, was shot in February 1989 shortly after Stakeknife left the house where he was being held by the nutting squad, which had obtained a confession following a violent struggle.
Panorama said Stakeknife told his own Army handlers Fenton would not survive but no action appeared to have been taken to prevent the killing.
Some nutting squad victims were said not to have been properly investigated to protect Stakeknife's cover.
Northern Ireland's Director of Public Prosecutions Barra McGrory told the programme: "What we're talking about here are almost parallel processes.
"We have one in which there's a police investigation, but all along there is an entirely secret dimension to these events.
"Now that drives a coach and horses through the rule of law.
"It means that the people who carried out these murders were not properly investigated or brought to justice, so for me that is an appalling vista."
In 2003 Stakeknife was widely named as west Belfast man Freddie Scappaticci but he has always strongly denied the allegation.
Almost 50 detectives are working on the Stakeknife investigation and have uncovered significant new evidence.
The investigation was launched after Mr McGrory referred the multiple allegations to the Police Service of Northern Ireland (PSNI).
Mr Boutcher has said some families may learn for the first time that their relatives' deaths were connected to the Army's prized agent.
Panorama will be broadcast on Tuesday night.