A former policing watchdog who investigated the Omagh bombing has said it could have been prevented.
Baroness Nuala O'Loan called for a public inquiry into the worst single atrocity of the Northern Ireland conflict and seriously questioned the handling of security force intelligence.
Later on Wednesday, relatives will mark 20 years since the August 1998 dissident republican blast which killed 29, including a woman pregnant with twins.
Baroness O'Loan said: "My view now is that it could have been prevented."
She said the various intelligence services could have worked in a more cohesive way.
On August 4 1998, 11 days before the bombing, the Royal Ulster Constabulary (RUC) received an anonymous telephone call warning that there would be an "unspecified" terrorist attack on police in Omagh on August 15 1998.
The force's Special Branch, which handled intelligence from agents, took only limited action on the information and a threat warning was not sent to the sub-divisional commander in Omagh, an investigation by Baroness O'Loan when she was police ombudsman found.
A RUC review concluded in 2000 that the information should have been passed to the commander.
Baroness O'Loan told the BBC: "If that had been conveyed to the sub-divisional commander in Omagh he could have just set checkpoints up around the town and the effect of that could have been to drive the bombers to abandon their bomb."
She said the intelligence services were tracking the movements of the car containing the bomb from the Republic of Ireland.
"What we do need now is an inquiry, a full public inquiry to find out why this happened and how it could have been prevented.
"I am not yet convinced that the way in which we handle intelligence across the UK is adequate to secure maximum information."
The massive car bomb ripped through the Co Tyrone town just months after the 1998 Good Friday peace agreement was supposed to have largely ended violence.
A splinter dissident republican group opposed to the peace process, the Real IRA, was blamed for the carnage after inaccurate warnings meant police evacuated shoppers towards the bomb site.
Twenty years later, nobody has been convicted of murder and relatives of the dead will gather on the spot on Wednesday afternoon for a short ceremony led by the Omagh Churches Forum.
It will be followed by the ringing of a bell 32 times to reflect the 31 lives lost and an additional peal to remember all who have lost their lives through similar atrocities.
The victims included Protestants and Catholics, tourists from Spain and others on a day trip from the nearby Republic of Ireland.
One of the biggest police manhunts in history unfolded, but criticism of the police investigation led to unfulfilled calls for a public inquiry.
Two people faced prosecution for murder but were not convicted.
Twenty years on, the Stormont power-sharing administration, which was a centrepiece of the Belfast Agreement, is suspended with little sign of its restoration.
Dissident republicans continue to pose a serious threat to life, primarily to members of the security forces.
Wednesday's commemoration will begin at 2.55pm at the site of the bombing in Market Street.
The bell will stop tolling at 3.10pm, the time the bomb exploded.
The ceremony will feature a song, and then people will be offered a flower petal to scatter into a river or a pond at a nearby memorial garden.