The soldier who killed a 41-year-old man in Londonderry in 1971 should have been charged with murder, the chief of the RUC in the city said at the time.
A report by Chief Superintendent Frank Lagan said the shooting of William McGreanery was unlawful.
The soldier who shot him, a member of the 1st Battalion Grenadier Guards, said Mr McGreanery had been armed.
The Historical Enquiries Team found that he "was not carrying a firearm and he posed no threat to the soldiers".
The information was revealed by Mr McGreanery's family after an investigation by the HET into Mr MrGreanery's death at the junction of Eastway, Lonemoor Road and Westland Street on 15 September 1971.
Soldier 'A' was never prosecuted on the advice of the attorney general, who said "whether he acted wrongly or not, the soldier was at all times acting in the course of his duty".
Mr McGreanery was one of a group of men who were walking past an army observation post when he was shot.
An excerpt of the HET report released by the McGreanery family said: "An examination of the original case file reveals that there were clear doubts over the veracity of soldier 'A's account yet nothing was done to challenge it or investigate further."
It commented on the "very real" threat against soldiers at the time, 49 were killed in 1971 and that when questioned about the incident soldier 'A' has said he had petrified.
"Thirty-eight years later, the soldier still maintains his original account of events during that evening," the HET report said.
"He does accept however, that he made a mistake, albeit an honest one."
The nephew and niece of Mr McGreanery, Billy McGreanery and Marjorie Roddy, said engaging with the Historical Enquiries Team had been a "difficult and painful process".
"Justice should have been done 39 years ago and it wasn't. It's a great pity that the information has come much too late for those who were closest to Billy," they said.
Foyle MP Mark Durkan said he was shocked that the report expressed so clearly the view that "soldiers were effectively immune from prosecution for murder for killings carried out while on duty".
"I am meeting An Taoiseach Brian Cowen TD this afternoon and I will present him with a copy of the HET report," he said.
"I will also be seeking to register and record these findings in parliamentary terms at Westminster and secure the redress that the McGreanery family deserves."