Republicans believe the IRA operation to attack Loughgall RUC station 30 years ago next week was compromised from an early stage.
However, it is not clear whether a single well placed informer was responsible for providing the information or if it was supplied by several lower level moles which was then pieced together by Special Branch.
Six SAS soldiers and three police officers were placed in the station ahead of the attack.
Had the RUC or British army known there was going to be a bomb attack it is unlikely their safety would have been put at such obvious risk.
In the days leading up to the attack information was received by the RUC which suggested an operation was being planned at Loughgall.
Then RUC Chief Constable Jack Hermon would then have called in the British army.
It is understood the security forces insist intelligence received about the attack did not contain some key details, including full details of who was involved.
Some republican sources suspect that several threads of information may have been drawn together that indicated the IRA was planning something and that Loughgall could be the target.
It is believed that the RUC had prior warning that two of the IRA’s most experienced members in east Tyrone were staying in a safe house in Ardboe in the days leading up to the attack.
Jim Lynagh and Padraig McKearney were both seasoned IRA men who had been on the run at the time.
This information is said to have been supplied to the police by a republican sympathiser, who it is claimed made her home available to the IRA.
Informed sources say the woman later admitted to the IRA that she became an informer after she was caught shoplifting in Cookstown.
It is understood that after being compromised she was told by the RUC to make herself known to a senior republican in the Ardboe area and allow her house to be used by republicans.
She was later abducted by the IRA from her home but rescued minutes later by the RUC.
Two men were later charged in connection with the kidnapping, including a brother of one of the men killed in Loughgall.
The charges were later dropped after the RUC informer did a deal with the IRA to retract her testimony.
It was later reported that she denied being an informer.
Sources say several other people were also questioned about the attack by the IRA.
After the ambush it emerged that there had been warning signs that those involved may have been under surveillance.
Informed sources say there were suspicions that component parts of the bomb were disturbed after they were left unattended during transport.
It is understood concern was also raised after a stranger was spotted in a phone box close to where members of the bomb team had gathered in advance of the attack.
