On Remembrance Sunday in November 1987, the IRA bombed the commemoration parade in Enniskillen, County Fermanagh. It was one of the most notorious attacks of the "Troubles".
All 11 people killed were civilians. There was worldwide condemnation.
For the IRA, the attack was a tactical disaster of monumental proportions that seriously damaged the republican cause and led to a serious internal debate about the way forward.
Ironically, Enniskillen was one of the seminal events that ultimately led to today's remarkable political settlement.
Many have long thought the bombing must have been an unauthorised, one-off operation by a local unit, believing it to be inconceivable that the IRA would mount such an attack on civilians as they remembered their dead.
It was nothing of the kind.
Three IRA active service units are believed to have been involved - one in the north (Fermanagh) and two in the south (Donegal and Monaghan).
A second attack on another Remembrance Day parade in the border village of Tullyhommon, in Fermanagh, was also planned but the bomb failed to go off. Clearly the two attacks were co-ordinated.
At the time, local IRA units were given a degree of operational autonomy but attacks of this magnitude against such sensitive targets would not have been carried out in isolation.
'Risk worth taking'
Detective Chief Superintendent Norman Baxter, who investigated Enniskillen and has studied contemporaneous intelligence reports, says that prior to the bombing there were deliberations at a very senior level within the IRA.
"The calculation was taken as to the number of casualties they could inflict on the civilian population against the number of casualties they could inflict on members of the security forces. And they decided that the risk was worth taking," he said.
"The civilians were collateral to the bomb but they were prepared to accept the number of casualties."
The question is: at what level was the attack authorised?
The body that oversaw and directed operations in the north was the IRA's Northern Command.
In the hours after the bombing, my sources say that McGuinness travelled to Fermanagh to question members of the local unit
Det Ch Supt Baxter said: "Northern Command had a knowledge of the attack prior to it taking place. They didn't stop it".
British and Irish security sources on both sides of the border have each independently told me that Martin McGuinness, now Northern Ireland's deputy first minister, was the leading figure on Northern Command at the time of the attack.
McGuinness told me that he had not been a member of Northern Command and he had had no knowledge of Enniskillen.
Intelligence reports record that three days before the bombing, McGuinness was stopped in a car by Irish police on the Donegal border. He was with three members of the IRA who were based in the south.
The subsequent intelligence assessment was that McGuinness was going to be briefed about the Remembrance Sunday attacks.
In the hours after the bombing, my sources say that McGuinness travelled to Fermanagh to question members of the local IRA unit to find out what had gone wrong.
McGuinness said that if he did go, it would have been in his Sinn Fein capacity.
Reports also indicate that the day after the bombing, he went to see the officer commanding the IRA's Donegal unit for the same purpose.
Finally, in the wake of the bombing, Gerry Adams and another senior IRA figure are reported to have discussed declaring an IRA ceasefire to try to mitigate the political damage, but McGuinness was against the idea.
Adams and McGuinness are said to have fallen out over the suggestion.
'Hard to stomach'
Significantly, many of those who were victims of the bombing now praise the journey McGuinness has made from "war" to peace.
Joan Wilson, whose husband Gordon so movingly described saying goodbye to their daughter Marie as she lay dying in the ruins, said: "I regard him as a good politician.
Watch a clip from Age of Terror: 10 Days of Terror
"I'm sure he has to learn a lot. We all learn from experience. But it was a big step for him too and I wish him well."
Stephen Gault saw his father lying dead in the rubble. "It was as if you took the top off an egg where his skull had just been ripped open," he said.
He has now reluctantly come to terms with McGuinness's transformation.
"It's hard to stomach him being the deputy first minister, but I think having peace in Northern Ireland is the best thing that ever happened.
"It's a hard pill to swallow, but I'd rather be where we are now than back in the Troubles."
Peter Taylor's The Age of Terror is broadcast on BBC Two at 2100 BST on Tuesday 22 April.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.