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For 20 years, senior Sinn Fein member Denis Donaldson led a secret life as an informer for MI5, the RUC and PSNI.

Then in April 2006, he was told by police that the media were about to expose his role as an agent and he fled to a remote cottage the family owned in Donegal.

He was shot dead in the cottage a short time later.

Three years after he was killed, the Real IRA said they were responsible.

But Denis Donaldson's family have alleged that police officers who knew about his secret role may have exposed him as an agent and contributed to his death.

They complained to the previous Police Ombudsman, Al Hutchinson, and asked for an investigation into the conduct of police officers who knew about his life as an informer.

Three years ago, in February 2010, he wrote to the family, saying his office had conducted "significant enquiries with the PSNI".

The letter went on to say that "no police misconduct has been identified" and declared that the matter was now considered closed.

The current ombudsman, Michael Maguire, has now confirmed that a new investigation has been launched.

In a statement, he said the decision was taken because of new information of which his office was previously not aware.

The statement is believed to be a reference to information uncovered by a ´óÏó´«Ã½ Spotlight programme in October 2011 which revealed serious failings in the investigation.

The ´óÏó´«Ã½ revealed that the original investigation was declared closed without investigators speaking to a Special Branch agent handler who used the name Lenny.

It was a phone call from Lenny that sent Denis Donaldson fleeing to Donegal from his west Belfast home and he told his family he remained in contact with Lenny while in hiding there.

His family believes the agent handler may have the answers to what happened.

The ´óÏó´«Ã½ also established that Mr Donaldson had been writing a journal while living in Donegal.

It was removed by garda officers investigating his death.

His family believe it could hold details of his life as an informant and potential clues about who killed him and why.

The family was initially told they would be given the journal when Mr Donaldson's personal belongings were returned to them, but were later told it was being retained "for security reasons".

Those working on the first ombudsman investigation were not aware of the existence of the journal.

After those revelations, solicitors acting for Denis Donaldson's family wrote to Al Hutchinson asking him to withdraw his finding that no police officer was guilty of wrong doing.

They also wrote to PSNI Chief Constable Matt Baggott expressing their rejection of the findings and their complaint was then referred back to the ombudsman's office.

In a statement on Wednesday, the current Police Ombudsman confirmed that his office is "looking at allegations about police conduct in connection with events surrounding the death of Mr Donaldson".

Michael Maguire added: "The office had previously looked at these allegations.

"Since then, members of Mr Donaldson's family have brought new information to the office which they were previously not aware of."

It is understood the new investigation has already started and that the Police Ombudsman will ask the garda for access to the journal, and seek to interview the Special Branch agent handler known as Lenny.

Mr Donaldson's family have welcomed the decision to launch a new investigation.

In a statement, they said they would fully co-operate with investigators and expect the PSNI to do the same.

The family urged any former or current PSNI officers with information about what happened to come forward.