The 1998 explosion killed 29 people and unborn twins
A former secretary of state for Northern Ireland has said the Omagh bombing in 1998 shaped the way he carried out his job.
Peter Mandelson was speaking at the launch of a book about the atrocity and the families' fight for justice.
Last month, a judge ruled that real IRA leader Michael McKevitt and three others were responsible for the bomb.
Lord Mandelson said the explosion, which killed 29 people, had given him "huge energy" to do his job.
He was one of the supporters of the families' campaign for justice and helped raise money for the multi-million pound civil case.
"In a very deep way and in a very personal way, it (the bomb) changed how I approached my job," he said.
Michael McKevitt is serving 20 years for directing terrorism
"But it also clarified what I was doing and why I was doing it and where I and, all those who I worked with, needed to end up with the people of Northern Ireland."
The book, Aftermath: The Omagh Bombing and the Families' Pursuit of Justice, was written by journalist and broadcaster Ruth Dudley Edwards.
It tells the story of the bombing, how the families coped with their loss and chronicles their civil case which culminated last month.
Ms Dudley Edwards, who has been working on the book since 2003, said it was the most difficult piece she had ever written.
"First, because it took forever, secondly because it was so hard to do well and to do it right, and because at times I thought there would not be a story to write about," she said.
"But I am the most proud of this book because I feel it honours these people I so much respect."
Ms Dudly Edwards said the civil case had tested the relative's patience.
"We all thought the case would come to court within a year and it took another five years to come to court," she said.
"And the law moves so slowly and so frustratingly, the delays were utterly terrible. It was mind numbingly awful the progress of the law."
Godfrey Wilson, who lost his daughter, Lorraine, in the bombing, said other families could learn from their experience.
"I'm delighted in the civil case the way it went and I'm hopeful... we'll go on to fight against terrorism. We need to fight against terrorism," he said.
"Terrorism hurt me and my family severely. To lose Lorraine was a terrible, terrible atrocity to myself and my family."
The book marks another chapter in the Omagh story - but the families believe the ending will only come when a criminal case is successful.
Victor Barker, who lost his son, James, said it was important for other people to read about the suffering caused by terrorism.
"I hope that it will lead to these people being exiled in their own communities and completely rejected by all the communities in Northern Ireland," he said.
"There is always the hope of a criminal prosecution one day. The families are determined to make sure that their cause and their fight goes on as long as it possibly can".