In a cruel twist of fate, Detective Chief Inspector Philip Marshall was probing the activities of the dissident republican cell that had grown out of the IRA gang that murdered his police officer brother more than a decade earlier.

Dad-of-two Constable Michael Marshall was ambushed and his heavily armoured car riddled with gun fire in an IRA attack in south Armagh in October 1989.

The 25-year-old was driving a patrol car through Beleeks, near Camlough, when gunmen opened fire using a heavy calibre machine gun mounted on the back of a lorry.

The armour-plated Sierra was riddled with more than 60 rounds and exploded into flames when the petrol tank was ruptured.

No one was ever charged with the murder, carried out by an IRA gang based in south Armagh - many of whom split from the Provos behind Real IRA leader Michael McKevitt in 1997 and who were involved in the Omagh bomb plot and other dissident attacks.

Last week his brother Philip's conduct was savaged by a Crown Court judge who branded him a liar after he told "untruths" in the witness box during the trial of acquitted Omagh bomb defendant Sean Hoey.

Marshall had given evidence in relation to a mortar find, but, the Crown Court heard, had lied about wearing protective clothes at the scene - photos taken at the time showed that he had not been wearing the protective clothes, as he claimed.

In his scathing judgment delivered last week, Mr Justice Weir said Marshall had been involved in "deliberate and calculated deception" and added that it was "impossible for me to accept any of the evidence of either witness since I have no means of knowing whether they (Marshall and scenes of crime officer Fiona Cooper) may have told lies about other aspects of the case that were not capable of being exposed as such".

DCI Marshall, who is still on duty, is currently under investigation by the Police Ombudsman after Mr Justice Weir referred transcripts of his evidence to Ombudsman Al Hutchinson's office.

Sunday Life attempted last week attempted to contact DCI Marshall, but was unable to do so.

A police spokesman said: "We do not comment on individual officers."