Detective Chief Superintendent Eric Anderson, who is heading the inquiry, says at least six people, including one from Omagh, were suspected of involvement in the atrocity.

Twenty-nine people were killed and hundreds injured when a huge bomb went off in the centre of the County Tyrone town on 15 August.

Eric Anderson: "We know who they are"

Mr Anderson said members of the Real IRA in County Armagh and across the border in Counties Monaghan and Louth had planned the blast.

List of suspects

He said although his team had drawn up a list of suspects they were struggling to build a case strong enough to convict them.

Mr Anderson told The Times: "We have a fair idea who did it. We probably know about half a dozen who were involved.

"We are trying to work out their actual roles. The problem is producing a case that convicts them. It's one thing to know who did it and another to produce evidence to put them away."

Twenty people have been arrested and questioned about the bombing but none have been charged.

The veteran RUC detective, who has investigated about 300 murders, including the 1978 Warrenpoint massacre in which 18 soldiers were killed, is optimistic about solving the Omagh case.

Mr Anderson, who was on the verge of tears at a news conference last month when he reflected on the bombing, said: "It's a long, protracted process involving many avenues of investigation and we're not there yet."