There is no evidence to link one of the two men jailed for murdering Constable Stephen Carroll to any role in the killing, the Court of Appeal has been told.
Constable Carroll, 48, was shot after responding to a 999 call in Craigavon, in March 2009.
Brendan McConville and John Paul Wootton are both appealing against their murder convictions.
Wootton’s counsel said the quality of forensic science used by the prosecution was “lamentable”.
McConville, 42, of Glenholme Avenue, Craigavon, is serving at least a 25-year sentence for the murder. Wootton, from Collindale in Lurgan, received a minimum 14-year term.
Prosecutors say that Wootton’s car was parked close to the scene of the attack and drove off within minutes of the killing.
Gun residue was discovered on a coat found in the vehicle, which was linked to his co-defendant.
But Arthur Harvey QC insisted that no role in the shooting was ever attributed to Wootton.
“There was not simply a dearth, but a total absence of evidence to connect the defendant to any specific act relevant to the murder of Constable Carroll,” he said.
Mr Harvey argued there was nothing to confirm the coat had been used on the night of the shooting.
He claimed the scientific evidence was significantly reduced by deficiencies in the research.
Recalling criticisms raised at the original trial, the barrister continued: “The quality of work from the forensic scientists representing the prosecution in this case was lamentable.”
Earlier in the hearing, it emerged that lawyers for McConville are to seek to have former prison ombudsman Pauline McCabe and former Maghaberry jail governor Steve Rodford called to give evidence at the appeal.
An investigation by Ms McCabe into the discovery of a note containing Mr Rodford’s personal details in McConville’s cell in 2009 concluded that it was probably planted by a prison officer.
Mr Rodford resigned a short time after the note was found.
The appeal continues.