A new inquiry from Northern Ireland's Police Ombudsman has found that Special Branch officers withheld information from detectives investigating the Omagh bombing in 1998.
Ombudsman Dr Michael Maguire said the failure to pass on details of telephone numbers used by the Real IRA hampered the initial investigation, but could not have prevented the attack.
Twenty-nine people, including a woman pregnant with twins, died when the car bomb ripped through the Co Tyrone market town on 15 August 1998.
It was the worst single atrocity of the Troubles.
Dr Maguire said the Royal Ulster Constabulary Special Branch had not provided all intelligence because of its interpretation of the law at that time, which he added was reasonable in the circumstances.
The Ombudsman's report outlines the findings of an investigation which focused on certain intelligence obtained between 15 August and September 1998, held by police.
Dr Maguire said: "I believe we had unfettered and unrestricted access to all the relevant documentation held by the PSNI."
He also found no evidence police had information which, if acted upon, could have prevented the Omagh bombing.
Dr Maguire said the RUC 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 bombing.
In 2001 former ombudsman 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 of State 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.
