Here are some of the key events in the Omagh families’ long quest for justice for their loved ones.

1998

In September, a month after the bombing, the RUC and Garda arrest 12 men in connection with the atrocity. They subsequently release all of them without charge.

1999

Seven men are arrested in a joint RUC-Garda operation.

Colm Murphy is charged with conspiracy to cause an explosion likely to endanger life or cause injury. The then 48-year-old, from Ravensdale, Co Louth, is also charged with membership of an unlawful organisation – the so-called Real IRA.

2000

At the inquest into the deaths of the 29 victims, coroner John Leckey says he will press the courts to prosecute the bombers with the destruction of unborn twins. Avril Monaghan had been pregnant with twins when she was killed. Mr Leckey says he is in no doubt that 31 people were killed in the bomb and that he will write to the Director of Public Prosecutions to ask him to consider charging anyone apprehended for the bombing with child destruction.

2001

A report by the Police Ombudsman finds the RUC Special Branch failed to act on prior warnings and condemned the RUC’s investigation of the bombing.

2002

Colm Murphy is found guilty by the Dublin Special Criminal Court of conspiracy to cause the Omagh bombing. He is jailed for 14 years.

2003

Alleged Real IRA leader Michael McKevitt, from Blackrock Co Louth, is found guilty of directing terrorism and membership of an illegal organisation at Dublin’s Special Criminal Court. He is sentenced to 20 years.

2005

Sean Hoey, of Molly Road, Jonesborough, Co Armagh, is charged with the murder of the 29 people killed in the Omagh bomb. He is the first person to face a murder charge in relation to the attack.

Murphy’s conviction in the Irish courts is overturned and a new trial ordered.

2007

Hoey is found not guilty at Belfast Crown Court of 58 charges, including the murders of 29 people in the Omagh bombing. Clearing Hoey, the judge criticises police witnesses for “deliberate and calculated deception” during the lengthy trial.

2008

The families of some of the victims of the bomb begin a landmark civil case, suing five men they claim were involved.

A memorial garden is opened in Omagh to remember the victims of the blast, as well as a monument on the site where the bomb exploded.

2009

The judge in the civil trial rules McKevitt, Murphy and two others – Liam Campbell, from Dundalk, and Seamus Daly, from Monaghan – were all liable for the Omagh bomb. He orders them to pay a total of £1.6 million damages to 12 relatives who took the case. A fifth man, Seamus McKenna from Dundalk, is cleared of liability for the bombing.

2010

Murphy is cleared following a retrial after interview evidence from the Garda is ruled inadmissible.

2011

McKevitt and Campbell lose their appeal against the civil trial verdict. Murphy and Daly both win their appeals.

2013

Murphy and Daly are both found liable for the Omagh bombing after a civil retrial.

McKenna dies after falling off a roof in Dundalk.

Northern Ireland Secretary Theresa Villiers rules out a public inquiry, insisting a fresh investigation is unlikely to reveal anything more about the attack.

Michael Gallagher, whose son Aiden died in the bombing, takes legal action against Ms Villiers’s decision.

2014

Daly is charged with the murders of 29 people in the Omagh bombing.

2016

The prosecution case against Daly collapses. The Public Prosecution Service decides there is no reasonable prospect of conviction after a key witness contradicted his own previous testimony.

A bid by Campbell and McKevitt to overturn the civil ruling that found them liable for the Omagh bomb is rejected by the European Court of Human Rights.

2017

Relatives of Omagh bomb victims sue PSNI Chief Constable George Hamilton for investigative failings they believe let the killers escape justice. The bereaved families issued a writ against the chief constable seeking damages and a declaration their human rights have been breached

2021

McKevitt dies.

Ruling in Mr Gallagher’s judicial review, a judge recommends the UK Government carries out an investigation into the Omagh bombing, and urges the Irish Government to do likewise, after finding “plausible arguments” that there was a “real prospect” of preventing the atrocity.