Police Special Branch in Northern Ireland was aware of intelligence information on the Omagh bomb soon after the atrocity, the former ombudsman claimed today.
But information was not immediately passed on to investigating officers who spent months combing mobile phone records in the hunt for the Real IRA bombers, ex-police complaints head Nuala O'Loan added.
The perpetrators are believed to have used phones to communicate on their way to Omagh with the device.
Dame Nuala gave evidence to a committee of MPs at Westminster.
"I am satisfied that material was passed to Special Branch," she said.
She talked about the reaction of secret service monitors.
"It triggered, and I am speculating, that the security services would be alarmed and distressed by the explosion and would immediately decide to search to see if they had anything which would be relevant, anything that had come in in the past hour or so."
A total of 29 people and two unborn babies died in the August 1998 blast in the Co Tyrone town.
It was the worst single atrocity and Ms O'Loan, who left as Ombudsman in 2007, drew up a report which heavily criticised the Royal Ulster Constabulary's probe into the crime.
She told the Northern Ireland Affairs Committee investigation opportunities could have been availed of.
"My understanding is that if you can get access to someone immediately after the bomb explodes...my understanding is that there's information to suggest that the bombers themselves were taken aback at what happened, that they didn't intend to kill all those people," she said.
"If you were then able to arrest them and bring them in it is possible you might have a much more coherent and complete conversation.
"It also would enable you to go to their homes and search the homes."
She said there was a five-month delay in some cases as detectives combed through all the phone records in the area.