A High Court judge in Northern Ireland says there was a "real prospect" to prevent the Omagh Bombing in 1998.
Mr Justice Mark Horner is recommending the single most deadly attack of the Troubles should be investigated by the UK and Irish governments.
29 people, including a woman pregnant with twins were killed on August 15th 1998 by an attack in county Tyrone, which was claimed by the Real IRA.
It was the worst single atrocity throughout the Troubles.
This legal challenge to the UK Government's decision not to hold a public inquiry into the Omagh bombing was launched by campaigners back in 2013.
The case concluded more than two years ago
Mr Justice Mark Horner recommended today that the UK Government is to carry out a human rights compliant investigation into what happened in the aftermath of the incident.
Despite not having powers over the Republic, the judgement also recommended the Irish Government carry out a similar investigation.
The UK's Secretary of State for Northern Ireland Brandon Lewis has said the British Government will consider the recommendations.