Denis Donaldson was shot dead in a remote Donegal cottage on April 4, 2006.
The Belfast man had admitted, just months before hand, that he had been a British spy at the very heart of the republican movement for 20 years.
Suspicion originally fell on the Provisional Irish Republican Army (PIRA) for the murder, but Belfast Telegraph Political Editor Suzanne Breen could later reveal that the culprits were from the dissident republican group, the so-called Real IRA.
How did she get that story, who was Denis Donaldson and how did his life as an informer unravel? She told Ciarán Dunbar the full story.