Police have dismissed the Bain case "slam dunk" as an "interesting theory" which has been taken out of context.

Assistant Commissioner Malcolm Burgess turned the focus from Robin Bain back to his son David Bain, who was convicted and imprisoned for 13 years for killing his family in 1994.

He said the only fingerprints on the murder weapon belonged to David Bain and his brother Stephen, who struggled before being killed.

The fingerprint evidence has been a bone of contention, with police saying at trial it was human blood while opposing experts have said it is either animal blood or not blood.


There was also a cluster of other, unidentifiable fingerprints.

The police response followed claims by experts on TV3's 3rd Degree show that parallel markings on Robin Bain's thumb were consistent with him firing a gun that morning. The marks - not noticed in the 19 years since the murder - matched those made by gunpowder residue when loading the magazine of a rifle shortly after it had been fired.

Mr Burgess said further study would be made of fingerprints of Robin Bain taken after his death. He said fingerprints from Robin Bain which showed clear patches on the thumb suggested cuts from "handyman" work. "Had these been powder marks or smudges as claimed, we would expect to see a complete fingerprint image."

David Bain's advocate Joe Karam said the police claim was destroyed by studying the photographs of Robin Bain's thumb. "Where are the straight lines?" he asked. "The area where they suggest there is some scar tissue forms a triangle. This is typical police smudging of the real issue."

He said there was no record of any cuts being found in Robin Bain's thumb during the autopsy. He also said forensic photographer Peter Durrant, a former police expert witness, had studied the image closely and found the marks were not cuts.

"Anybody on their computer can blow up this print and see the lines we're talking about - they're not cuts."

Mr Karam said the new evidence came on top of evidence at the second trial in 2009 which showed David Bain was innocent. He said he should be pardoned. Prime Minister John Key said a pardon required a high standard of proof - it "has to be shown that someone else actually committed the crime".

Labour justice spokesman Andrew Little said Justice Minister Judith Collins had "buggered it up" and called for an independent system for handling compensation claims.

Green Party co-leader Metiria Turei attacked Mrs Collins' "unfair handling" of David Bain's compensation claim. The claim, stalled because of court action, was likely to increase the amount of money if an award was eventually made.