The Omagh bomb exploded on 15 August 1998, killing 29 people including a woman pregnant with twins.
No-one has ever been convicted of the atrocity.
In March 2016, victims' families said they had lost their "last chance" for justice after the latest criminal prosecution case collapsed.
The Public Prosecution Service withdrew its case against Seamus Daly, from Jonesborough, County Armagh, after a key witness gave inconsistent evidence.
None of the families have received any compensation.
BBC News looks back at the many legal twists and turns in the families' two-decades campaign for justice.
15 August 1998
A large car bomb explodes on a Saturday afternoon in the centre of Omagh, County Tyrone. The town's main street is crowded with shoppers and more than 200 people are injured. Twenty-nine of the victims, including a woman pregnant with twins, will die as a result of their injuries. In terms of the final death toll, it is the worst single atrocity after almost 30 years of the Troubles in Northern Ireland.
18 August 1998
A recently-formed dissident republican group, calling itself the Real IRA, claims responsibility for the bomb. In a statement, the paramilitary group says its targets were "commercial" and offers an apology to the "civilian" victims.
12 December 2001
The judgement and leadership of the head of the police in Northern Ireland during the Omagh bomb investigation is described as "seriously flawed". The comments come in a damning report by the Police Ombudsman Nuala O'Loan. She concluded "with great sadness" that the judgement and leadership of the then Chief Constable, Sir Ronnie Flanagan, and the assistant chief constable of the crime division, was "seriously flawed".
Later, Sir Ronnie said both he and the force were considering legal action to quash the report. In an emotional statement, Sir Ronnie said that if he believed the allegations in the report had been true "I would not only resign, I would publicly commit suicide".
22 January 2002
A dissident republican is found guilty of plotting to cause the Omagh bombing. Colm Murphy, then 49, is the only person charged in connection with the bombing. Murphy, a builder and publican orginally from south Armagh, had denied one charge of conspiring to cause an explosion likely to endanger life or cause serious injury to property between August 13 and 16, 1998. But three judges at Dublin's Special non-jury Criminal Court delivered a guilty verdict.
24 January 2002
The Police Service of Northern Ireland rejects a number of key allegations made in the ombudsman's report. Sir Ronnie Flanagan accepts that some mistakes were made, but insists that nothing could have been done to prevent the bombing.
6 August 2003
The alleged founder and leader of the Real IRA, Michael McKevitt, is found guilty of directing terrorism and membership of an illegal organisation in the Republic of Ireland. The businessman, from Blackrock in County Louth, is not charged over the bomb but is the first person to be prosecuted for directing terrorism. The Irish government introduced the new offence in response to the Omagh attack.
21 January 2005
Colm Murphy's conviction is ruled unsafe due to alleged irregularities surrounding evidence given by detectives at his trial. He now faces a re-trial.
23 January 2008
Sir Ronnie Flanagan, who was head of the police at the time of the Omagh attack, apologises to the families of the victims of the bomb. He said he was "desperately sorry" people had not yet been brought to justice.
26 May 2005
County Armagh man Sean Hoey is formally charged in court with the murders of 29 people in the 1998 Omagh bombing. The electrician, from Molly Road in Jonesborough, is the first person to face a murder charge in relation to the attack.
20 December 2007
Sean Hoey is found not guilty of 58 charges, including the murders of 29 people in the Omagh bombing. Clearing Mr Hoey, the judge criticises police witnesses for "deliberate and calculated deception" during the 10-month trial.
7 April 2008
The victims' families begin a landmark civil case, suing five men they allege were involved. The case breaks new legal ground, and is believed to be the first time anywhere in the world that alleged members of a terrorist organisation have been sued.
8 June 2009
The judge in the civil trial rules that Michael McKevitt, Liam Campbell, Colm Murphy and Seamus Daly were all liable for the Omagh bomb. He orders them to pay a total of £1.6m damages to 12 relatives who took the case. A fifth man, Seamus McKenna, is cleared of liability for the bombing.
24 February 2010
Colm Murphy, the only man jailed in connection with the bombing, is cleared following a retrial. The judge said interview evidence from members of the (gardai) Irish police was inadmissible.
7 July 2011
Michael McKevitt and Liam Campbell lose their appeal against the civil trial verdict. Colm Murphy and Seamus Daly both win their appeals but will later face a civil retrial.
20 March 2013
Seamus Daly and Colm Murphy are both found liable for the Omagh bombing after a civil retrial.
12 September 2013
The then Northern Ireland Secretary, Theresa Villiers, says she has decided not to hold a public inquiry into the circumstances surrounding the Omagh bombing, adding that she did not believe there were sufficient grounds to justify a further inquiry beyond those that have already taken place.
14 July 2013
Seamus McKenna, who was acquitted in the civil action taken by relatives of the bomb victims, dies age 58. His death is a result of injuries he sustained in a fall while repairing a roof at a school in Dundalk, County Louth. A major security operation is put in place for his funeral four days later.
10 April 2014
Police in Northern Ireland charge Seamus Daly with the murders of 29 people in the Omagh bomb attack. They also charge him with an attempted bomb attack in Lisburn, County Antrim, that took place in 1998.
1 March 2016
The prosecution case against Seamus Daly collapses. The Public Prosecution Service decides there is no reasonable prospect of conviction after a key witness contradicted his own previous testimony. Mr Daly, who has always denied any involvement in the bombing, is released from prison.
29 September 2016
A bid by two men to overturn a landmark civil ruling that found them liable for the Omagh bomb is rejected by the European Court of Human Rights. Liam Campbell and Real IRA leader Michael McKevitt took their case to Europe, arguing that the civil trial in Belfast High Court had been unfair.
20 December 2016
Belfast's High Court allows some evidence connected to the 1998 Omagh bombing to be heard in secret. The government had applied for a "closed material procedure" (CMP) for a judge to examine whether public disclosure of information would be damaging to national security. After viewing a sample of sensitive documents, the judge ruled that a CMP would be allowed.
10 August 2017
It emerges that relatives of the Omagh bomb victims are to sue Northern Ireland's police chief for failings they believe allowed the killers escape justice. A writ is issued against the chief constable, focussing on what happened after the car bomb and why no-one has been convicted of murder.
3 July 2018
A legal challenge to the government's refusal to hold a public inquiry into the Omagh bombing is pushed back to 2019. It is reported that Michael Gallagher, whose son, Aidan, died in the attack, is taking legal action against former Northern Ireland Secretary of State Theresa Villiers. Proceedings were adjourned to February 2019 after issues of national security were raised in a closed session.