The families of the four victims killed in the 1982 IRA Hyde Park bombings said they are “devastatingly let down” after the trial of John Downey collapsed due to a “reckless” error made by police.

Downey (62), a convicted Irish Republican Army member, from County Donegal, received a “letter of assurance” in 2007 from the British police, despite the fact that there was an outstanding warrant against him. The letter stated that he was not wanted over the IRA attack.

The July 20, 1982 attacks represented one of the IRA’s biggest killings of British troops. They detonated bombs during public celebrations of the British military in both Hyde Park and Regent's Park. The bombings killed four soldiers and injured many others.

Downey was arrested in May 2013 at London's Gatwick Airport while he was en route to Greece. He had previously traveled to the United Kingdom.

On Tuesday, Justice Sweeney threw Downey’s case out having earlier heard from the defense, Henry Blaxland, QC, who warned of the political ramifications in Northern Ireland of pursuing the trial. He called the false assurance Downey received “not just negligent, it was downright reckless.”

Sweeney said during his ruling that the public interest in prosecution was “very significantly outweighed” by the public interest in ensuring that “executive misconduct does not undermine public confidence in the criminal justice system and bring it into disrepute,” and in “holding officials of the state to promises they have made in the full understanding of what is involved in the bargain.”

It is now being questioned why the Police Service in Northern Ireland did not attempt to rectify their mistake in 2007 when the letter to Downey was issued.

On July 20, 1982, the British troops were engaged in entertainment and ceremonial duties on South Carriage Drive , in Hyde Park. The bomb planted in a parked car was detonated by remote control as the troops moved towards Buckingham Palace.

The explosion killed Roy Bright, Dennis Daly, Simon Tipper and Jeffrey Young and injured other members of the Royal Household Cavalry. Seven horses were also killed.

Two hours after this attack a suitcase bomb close to Regent’s Park killed seven army musicians. Witnesses said the musicians were blown into the air. Twenty-two others were injured.

British police investigating the Hyde Park attack were led to Downey through fingerprints on parking tickets and two witnesses.

In 1989 an arrest warrant for Downey was issued, but it was decided not to seek his extradition from the Republic, in part due to the lack of strong evidence.

In 2007 Downey was one of the 187 “On the Runs” who sought clarification from the authorities in the wake of the Good Friday Agreement. He received a letter stating that he was not at risk of prosecution as part of a scheme run by the Northern Ireland police.

Downey’s lawyer Blaxand said “Sinn Fein impresses it is impossible to overstate the importance of the assurances given to the 187 people.”

He added, “Once the trust starts to break down the whole edifice starts to crumble.”

At the time of Downey’s arrest Sinn Fein MLA Gerry Kelly said, “The decision to arrest and charge him in relation to IRA activities in the early 1980s is vindictive, unnecessary and unhelpful. It will cause anger within the Republican community.”

He continued, "This development represents bad faith and a departure from what was previously agreed by both governments."

Downey was to be the third man to face trial for this crime.

The BBC reported that in 2012 a survivor of the IRA attack at Hyde Park, suffering from post-traumatic stress, fatally stabbed his children before killing himself. Ex-Army sergeant Michael Pedersen’s children Ben and Freya, aged seven and six, were found near Andover, Hampshire, in September. Pedersen (51) was a Household Cavalry rider on Sefton, the horse who famously survived the bombing.

A brother of one of the victims of the Hyde Park bombings speaks about the case collapse: