The 1998 explosion killed 29 people and unborn twins
Real IRA leader Michael McKevitt was responsible for the 1998 Omagh bomb, a judge has found.
Mr Justice Morgan made the ruling at the High Court in Belfast after a landmark civil case brought by some of the families of the victims of the atrocity.
Liam Campbell, Colm Murphy and Seamus Daly were also found liable for the attack. Seamus McKenna was cleared.
The 12 relatives were awarded more than £1.6m in damages for the attack.
Mr Justice Morgan also found the dissident republican organisation the Real IRA liable for the bomb in which 29 people and two unborn babies were killed.
He said it was clear that the bombers' primary objective was to ensure that the bomb exploded without detection, and the safety of those members of the public in Omagh town centre was at best a secondary consideration.
He said he was "satisfied that those involved in the planning, preparation, planting and detonation of the bomb recognised the likelihood of serious injury or death from its detonation but decided to take that risk".
Mr Justice Morgan said McKevitt "held and has always held a significant leadership role in the Real IRA,", and was heavily involved in the procurement of explosives at the time of the bombing.
Much of this was based on evidence obtained by an undercover FBI agent, David Rupert.
He also said he was satisfied that Campbell was a member of the Real IRA's Army Council in August 1998.
Vincent Kearney, NI Home Affairs correspondent
None of the men being sued has the capacity to pay out any kind of large-scale payment.
From the start the families made clear the civil action was a vehicle for putting as much information as possible into the public domain about the bombing and the men they claim were involved.
It is not a criminal case and no-one will be jailed.
The families' case was heavily reliant on records and traces on two mobile phones used by the bombers on the day of the attack.
The judge said the evidence proved both Campbell and Daly were in possession of the phones before and after the attack.
The case against McKenna was dismissed because it was based on hearsay evidence from his estranged wife, who was deemed an unreliable witness.
Michael Gallagher, who lost his 21-year-old son in the attack, said it was a "result better than we could ever have imagined."
"We have sent out an important message to terrorists and their victims around the world - you now have a way of challenging those who've murdered your loved ones.
"I think it is a tremendous moral victory for the families," he said.
Victor Barker, whose 12-year-old son James was killed in the attack, said he was delighted with the outcome.
"I never built my hopes up too much after what happened before, but I'm absolutely over the moon."
Mr Justice Morgan said it was clear from the reports he had read and the evidence of the plaintiffs that "the senseless and indiscriminate nature of this appalling outrage has deeply affected each of them".
"For many, the events are catastrophic and their lives will never be the same."
THE MEN SUED BY THE OMAGH FAMILIES
Michael McKevitt - Co LouthColm Murphy - Co Louth Liam Campbell - Co Louth Seamus McKenna - Co ArmaghSeamus Daly - Co Monaghan
No-one has ever been convicted in a criminal court of causing the deaths.
The only man to face criminal charges over the Omagh killings, 38-year-old Sean Hoey from Jonesborough in South Armagh, was acquitted in 2007.
The families brought the multi-million pound action in an attempt to bring fresh information about the atrocity to light.
Civil cases have a much lower burden of proof, with the judge reaching his verdict on the balance of probabilities. In criminal law, guilt must be proved beyond reasonable doubt.