The Serious Organised Crime and Police Act means someone accused of committing a crime can enter into a written agreement to become what's called an assisting offender and help the prosecution by giving evidence against other alleged criminals.

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"I know this decision will be disappointing to Mr O'Hagan's widow, family, friends and colleagues but the evidence that can be given by an assisting offender must be carefully evaluated and the test for prosecution applied on a case by case basis, " Mr McGrory said.

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The two other men, aged 27 and 25, were arrested on suspicion of assisting an offender.

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