Evidence from the Hyde Park bombing of July 20 1982 quickly led police to convicted IRA member John Downey.
But the opportunity to let a jury decide on whether he was guilty or innocent of the IRA attack has eluded authorities and families of the victims.
At the time, it was not considered by the Attorney General or police to be strong enough to secure a successful extradition from the Irish Republic.
Nevertheless, a warrant for Downey was issued by Scotland Yard and remained active for three decades in case he should come into the UK of his own accord.
The evidence centred around NCP parking tickets with Downey’s fingerprints on them, according to an overview of the case set out in his judgment by Mr Justice Sweeney.
Downey, 62, of County Donegal in Northern Ireland, has always strenuously denied he was involved in the bombing which killed four soldiers and seven horses, and injured 31 more people.
It was caused by a remote control improvised explosive device which contained 20-25 pounds of commercial high explosive with wire nails as shrapnel.
The bomb was hidden in the boot of a blue Morris Marina car parked in South Carriage Drive and detonated as the guard was passing en route to the changing of the guard.
The victims were Lieutenant Denis Daly, 23, Trooper Simon Tipper, 19, Lance Corporal Jeffrey Young, 19, and Squadron Quartermaster Corporal Roy Bright, 36.
The Marina had been bought the week before at a car auction in Enfield by a man with an Irish accent who gave false details, according to the court document.
The prosecution case was based on the fact Downey had been convicted in 1974 of being a member of the IRA, according to the judgment.
His appearance in 1982 was allegedly consistent with photofits and artist’s impressions created from three witnesses who reported two men carrying out reconnaissance in South Carriage Drive on June 30 and July 1 1982.
Furthermore, three of Downey’s fingerprints were found on a ticket dispensed when the Marina was driven into an NCP car park in Portman Square, London, on July 17 1982 and surrendered when the car was driven away the following day - two days before the bombing.
And two more fingerprints were found on the ticket dispensed when the Marina was driven into the NCP car park at the Royal Garden Hotel in Kensington, west London, at 6.39pm on Sunday July 18 1982 and driven away at 6.51am on Tuesday July 20, just four hours before the bombing, the judgment said.
Downey became a suspect when police allegedly matched his fingerprints on the Royal Garden Hotel ticket with prints taken by the Garda in the Irish Republic in July 1980.
A photograph of Downey was found from a “delicate source” and believed to match a photofit from one of the witnesses, the judgment said.
After he was arrested last year at Gatwick Airport, Downey was charged with the four murders and with causing an explosion.
Members of the victims’ families had sat in the public gallery as lawyers for the prosecution and defence argued over whether a trial should go ahead.
After Mr Justice Sweeney gave his judgment, throwing out the case, Downey declined to comment to reporters in Court One at the Old Bailey.