This week marks 20 years since the Sinn Fein official was outed. In the second of a three-part series, Allison Morris examines how an informer could hold vital information on his murder
Denis Donaldson pictured outside his Donegal bolt hole shortly after he was exposed as a informer
After being outed as an informer for more than 20 years, Denis Donaldson had been living in a cottage in rural Donegal when he was shot dead.
The unsolved April 2006 murder was back in the headlines this year after featuring in a libel case taken by Gerry Adams against the BBC.