Police intelligence officers withheld some information from detectives investigating the Omagh bombing in which 29 people were killed, a new probe has found.
Even though it could not have prevented the Real IRA attack on Omagh 16 years ago, the failure to disseminate the data - telephone numbers - detracted vital resources from the initial investigation, the Police Ombudsman said.
In a report published today, Dr Michael Maguire said: "The consequence was that the police investigation was required to invest substantial resources in analysing related records, resources that might well have been better utilised at that early stage of the inquiry."
Twenty-nine people, including a woman pregnant with twins, died when the 500lb car bomb ripped through the Co Tyrone market town on August 15, 1998. It was the worst single atrocity of the Troubles.
Dr Maguire said the RUC (Royal Ulster Constabulary) Special Branch had not provided all intelligence because of their interpretation of the law at that time.
"The view held by the relevant police officers was a reasonable one in the circumstances," he said.
The Ombudsman's office, which independently investigates police work, launched an inquiry after a report by a group of British MPs outlined remaining questions surrounding the bombing.
In March 2010, the Northern Ireland Affairs Committee called for a new investigation into whether intelligence relating to those suspected of the bombing was passed on to detectives investigating it and if not, why not.
The Police Service of Northern Ireland chief constable then ordered an independent investigation of a number of specific matters concerning how RUC Special Branch handled its intelligence and its relationship with Government Communication Headquarters (GCHQ) - a Government body which deals with intelligence and security matters.
The Ombudsman's report outlines the findings of an investigation which focussed on certain intelligence obtained between August 15 and September 1998, held by police.
A previous investigation by his office had focused on the use of other information held by police at the time.
In January 2009, Peter Gibson, who led the British government's review of intelligence matters related to the bombing, published his report in which he said Special Branch had acted in a cautious way in their provision of material to the team investigating the bombing.
Investigators from the Ombudsman's office spoke to a range of witnesses including Gibson; serving and retired police officers and officials from other Government agencies. They also looked at a substantial amount of intelligence and investigative material.
"I believe we had unfettered and unrestricted access to all the relevant documentation held by the PSNI," said Dr Maguire.
Like a previous report by the Police Ombudsman's Office, in viewing this particular material, Dr Maguire also found no evidence police had information which, if acted upon, could have prevented the Omagh bombing.
He said Special Branch provided detectives with details of public telephone kiosks from where the bomb warning calls were made as well as the identities of suspects.
However, telephone numbers were not passed to the detectives.
Dr Maguire concluded that Special Branch had acted "cautiously" in not disclosing all the intelligence available to the team investigating the Omagh bombing.
"I am satisfied this action was as a result of the interpretation by Special Branch of both what Sir Peter [Gibson]'s report called 'the strict conditions imposed by GCHQ' on the RUC and the legislative framework which prevailed at the time and which, for all intents and purposes, is still in place, albeit in a different legislative form."
In 2001 former Ombudsman Dame Nuala O'Loan carried out a report on Omagh into the police's handling of warnings received from an informer.
She concluded they would not have been enough to stop the bombing but check points could have been erected around the town if police had reacted to a separate anonymous caller about a planned gun attack.
In April, a 43-year-old man from the Republic of Ireland was charged with the murders.
Seamus Daly, from Culloville, Co Monaghan, was arrested by serious crime branch detectives.
Nobody has been convicted of murder at Omagh.
However, relatives of some of the victims brought a landmark civil action against five men they claimed were responsible.
Four of the five men were ordered to pay more than £1.5m in damages to the victims' families in a civil case.
Families are also engaged in a fresh bid for a civil case challenging Northern Ireland Secretary Theresa Villiers's decision to rule out holding a public inquiry into the case.
Michael Gallagher, who lost his 21-year-old son Aiden in the explosion, said the Omagh families would continue their fight for a full, independent public inquiry.
He said: "The Ombudsman's investigation was pretty narrow focussed but we always knew that was going to be the case because that was the instruction from the chief constable.
"The Secretary of State Theresa Villiers said the Ombudsman's report would answer all outstanding questions in relations to Omagh but it clearly has not.
"She was wrong in her assumption. There are questions still remaining.
"The families still want an independent inquiry that will examine all outstanding issues. That is the only way forward. We have had several narrow focussed investigations before and they do not tell the full story.