Firefighters and police officers search the rubble in the aftermath of the Omagh bombing in 1998

The last detective to investigate the Omagh bomb atrocity has claimed vital evidence may have been held back from the original police investigation for fear of endangering the IRA ceasefire.

On the 15th anniversary of the Real IRA massacre, former chief superintendent Norman Baxter alleged that Downing Street was warned by republican contacts that to bring the Army into south Armagh to make arrests in the immediate aftermath of the bomb could destabilise the ceasefire.

"The warning was given in the strongest terms," he told the Belfast Telegraph.

"I would support a public inquiry into the decision-making process following this call and whether it led to evidence in the hands of the intelligence services being withheld from the police investigation."

The Omagh bomb was the single worst terrorist atrocity of the Troubles. On August 15, 1998, the explosion in the centre of the town killed 29 people and unborn twins, causing massive outrage.

No one has ever been successfully brought to justice in a criminal court for the atrocity. One man, Colm Murphy, was jailed in 2002 in the Republic for the attack.

However, his conviction was later overturned, and he was acquitted at a retrial.

Another suspect, Sean Hoey, prosecuted in Northern Ireland, was acquitted in 2007.

A civil case by families of the victims resulted in the only legal victory in the aftermath of Omagh. Four men – Michael McKevitt, Liam Campbell, Colm Murphy and Seamus Daly – were held liable for the attack and the families were awarded £1.6m in damages. The case against one man, Seamus McKenna, was dismissed. He died last month.

To back his claims, Mr Baxter pointed to a 2008 BBC Panorama programme which revealed that GCHQ, the UK's electronic surveillance agency, had been tapping phones used in the attack.

Raymond White, a former senior RUC officer, confirmed that Special Branch had passed on an Irish Eircell mobile phone number to be tapped.

It had earlier been used in an attack in Newry and a failed attack in Banbridge. This phone was one of a number later used by the Omagh bombers and monitored by GCHQ, with MI5 oversight.

But back in Belfast and Dublin, RUC and Garda detectives spent months trawling through 6.4 million calls to try to identify likely numbers – the ones already identified by GCHQ – used by the bombers on the day of the attack.

Mr Baxter believes the reason for not handing over the information may have been a message British officials received from pro-peace process republicans the morning after the bombing.

"On the Sunday (August 16, 1998) Sinn Fein people contacted representatives from Downing Street who were involved in the peace process.

"They advised them that if the green army, the British military, went into south Armagh to make arrests, the ceasefire would break. That was very strongly put," said Mr Baxter.

He said the question is whether "following that contact, a decision was made not to release intelligence that would lead to arrests. That question needs to be answered in the context of an enquiry".

In another revelation, Mr Baxter also claimed that Dave Rupert, an MI5 and FBI spy within the Real IRA, was never asked to gather evidence on the bombing.

Mr Rupert was in the rival Continuity IRA at the time of the Real IRA attack on Omagh.

While in the CIRA, Mr Rupert, a near bankrupt trucker who posed as a wealthy American, warned his handler that a stolen car the organisation was holding in Letterkenny might be used to attack Omagh or Londonderry.

That was on April 11, five months before Omagh.

Belfast Telegraph