Press senior writer Martin van Beynen has been writing about the Bain case since 1997 and covered David Bain's second trial in Christchurch in 2009. He has made no secret of his view that David Bain killed his family in June 1994, and today gives his opinion on the TV3-aired evidence which Bain's supporters say removes all doubt about Bain's innocence.

Throughout the Bain saga, there has always been a sneaking suspicion that buried in the mountain of evidence relating to the killings is a piece of the puzzle which would end all argument. The clincher, the game-changer.

Before the trial in 2009, for instance, Bain's lawyers claimed to have startling new evidence which, in the event, never lived up to the prior publicity. The police thought they had slam-dunked the case when they believed they had detected David saying "I shot the prick" during his emergency phone call. The evidence didn't make it to the trial.

However, on Wednesday night TV3 claimed to have found that all-important puzzle piece - two marks on the right thumb of David's father Robin, marks apparently missed by the hundreds of professionals and amateurs who have pored over the case.

The programme contended the marks were made when Robin Bain loaded the magazine of the murder weapon, a .22 rifle, at some proximate point before he shot himself in the head in the lounge of the house. It said the lines came from burned gunpowder residue typically found on top of the magazine housing and which rubbed onto Robin's thumb.

Do I now need to issue a public apology to David Bain and Joe Karam? It's true I would be left with a huge amount of egg on my face if indeed the apparently new discovery proved David Bain's innocence. So I accept from the outset these comments have to be read in that light.

No apology will be coming from me. The latest revelation is certainly not the clinching piece of evidence the programme claims. It's not even particularly convincing.

For a start the marks don't even look much like the sort of powder deposits seen on the collection of Bain camp experts who participated in the tests by loading bullets into the rifle's clip.

If the marks on Robin's thumb are from the magazine they would have been deposited close to the moment Robin shot himself. So you would expect an imprint very similar to that left on the thumbs of the Bain experts. It's possible some of the residue was removed as Robin placed and held the rifle to shoot himself in a very odd way but, amplified, the lines on Robin's thumb look defined and crisp.

Unlike the marks left on the Bain camp's thumbs, the marks on Robin's digit are not parallel or soft in outline. They are also thinner and from what I can see, not even the same colour as the test marks.

If Robin was loading a number of bullets, as he must have according to the defence scenario, how come only one set of marks was left on his thumb? Did his thumb follow the same track every time?

We also need to remember that under the defence scenario, Robin, sometimes wearing David's gloves, must have handled the gun extensively. Yet the only fingerprints on the rifle belong to David Bain and his murdered brother Stephen. Neither did either of the two magazines - one in the rifle and one on the floor - have any of Robin's fingerprints.

The programme certainly highlighted, possibly without meaning to, the position of the magazine right next to Robin's hand. It was perched on its narrowest side.

The new scenario propounded by the TV3 programme would have Robin shooting his family and then waiting to just before David was due to come home from his paper-run to turn on the computer (so he could write his last message) and, before shooting himself, placing the spare magazine on its narrowest side on the carpet.

Then when he falls to the carpeted floor after the fatal shot, he conveniently lands with his hand right next to the magazine. It seems much more likely the killer placed the magazine on its edge right next to Robin's hand to make it look like a suicide.

Robin appears to have had only two main opportunities to have got the marks on his thumb from one or other of the magazines.

The first is when he was loading and reloading the magazine before and during the period when he went around the house executing his wife and children. Remember this was not a mad, distracted shooting.

On the defence scenario, Robin, after the killings, had to have changed his clothes and footwear, put his soiled clothes in a washing basket and had a bit of cleanup before he shot himself. This is because none of his family's blood was found on his clothes, hair or skin. So if the marks on his thumb were there before the change and clean-up, they had to have escaped being rubbed off or smudged.

It's also possible the marks were deposited just before Robin shot himself as he loaded the magazine again before the final exit. But given the gun's tendency to misfire why didn't he just put a bullet in the breech?

So what produced the marks if it wasn't the magazine? One thing the TV3 programme highlighted was the state of Robin's hands. They look pretty battered and bruised. The Bain camp would say the scabs and bruises come from Robin's violent struggle with his son Stephen but they are much more likely to have come from handyman Robin repairing his guttering on the weekend before the killings (on Monday).

The Whale Oil blog has raised the very real possibility they are the marks of someone who plays the guitar or banjo. Robin was an accomplished musician.

All these points highlight the principle that no piece of evidence in the Bain or another case should be looked at in isolation. The case was a textbook example of how circumstantial evidence can identify a killer and each strand needs to be seen in the light of the others.

To say the marks end the argument is laughable. As I have said many times, David Bain, if he wants to show he is innocent, needs to explain about 20 to 25 pieces of evidence which point to his guilt.

I would have a lot more faith in the TV3 programme if it had tried to be at least a little independent. The photographic expert the programme consulted was Christchurch-based Peter Durrant, a scientific and technical photographer who in 1985 developed a new photographic technique for analysing the visual effects of wear on the pile of carpets.

He is not a forensic scientist and has no expertise about firearms and residues on bodies. He also gave evidence as a defence expert for Bain in the second trial and was largely discredited, in my view, when it turned out the photographs he had produced to support a defence theory were out of the sequence in which they were taken.

An ESR scientist was at the test firing featured in the programme. Was he asked for his view?

As viewers, we also need to ask why an independent forensic scientist was not consulted before reporter Melanie Reid concluded emotively, the "game changer" had been found. TV3 bought the defence line on the evidence and did a one-sided job presenting it.

The programme, and the media coverage of the Bain case in general, also raises a wider issue.

Everyone would have noticed the new information was released first to Bain-friendly media. TV3 had the exclusive and The Herald had the first newspaper story. (TV3 also got the first patsy television interview with David Bain.)

Fairfax, which owns The Press, did not get a look in and neither did TVNZ. Both have published and broadcast material unfavourable to the Bain camp.

You also have to ask, if the information was so crucial, why wasn't it shown to the Minister of Justice first so she could get Crown experts to investigate.

It would be naive to suggest the media doesn't play games and doesn't get played itself. But the Bain case seems to be driving the media into throwing independence to the wind so it curries favour with one camp. The TV3 programme was in my view, a sad and painful night for New Zealand journalism.

I don't claim to be independent on the Bain case either, given I have nailed my colours to the mast. But at least it is out in the open.

The media is being played as part of the Bain camp's attempt to put pressure on the Government to pay compensation. So marks on a thumb, a game changer? Only if you ignore everything else.