A leading war crimes lawyer tells Channel 4 News the video apparently showing men taking part in executions in Sri Lanka is "astonishing evidence" and the United Nations must act.

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Last week, Channel 4 News screened new footage showing an alleged massacre of Tamil prisoners in Sri Lanka.

New evidence has since emerged over the possible identities of one of the victims as well as on which soldiers may be responsible. The video has been sent to the United Nations panel which is currently investigating whether to hold an international inquiry on what happened during the 26-year war in Sri Lanka.

Channel 4 News spoke to war crimes lawyer Julian Knowles of Matrix Chambers about the video and what could happen next.

Can I ask you what your initial view is of what this video, what we've learned from evidence contained in it, what it says to you about possible war crimes?

What you've got here is clear evidence of the execution of unarmed combatants or civilians. It doesn't matter which they are, they're both prohibited under the Geneva Convention and they are both ranked as what we call a grave breach of the Geneva Convention so they are crimes in international law.

And whether these are combatants or whether they're civilians, their hands are tied, they're blindfolded, there's no evidence of any weapons, so we clearly have executions here which are a crime in international law and one of the most serious crimes.

And does the fact that we now know pretty much exactly when it happened and roughly the location, does that have a bearing on a possible future trial?

The evidence that you've uncovered with this video, and what this video shows, is absolutely crucial in the forensic process because it allows you to identify the troops on the ground, the unit which they are from, their local commanders and the higher level commanders. Because if you know the region, the Sri Lankan government will know what troops it had and at what time. So because you know the date and because you know pretty precisely the location, the Sri Lankan government, if it had the will, would be able to identify who is responsible for these executions and that would obviously be crucial in any trial, if you want to put on trial both the soldiers who actually pull the trigger and the commanders who gave them the order to do so.

In the line of command responsibility, how high can it go?

It will go high. Strategies like this, the cold-blooded killings of civilians, is not the sort of decision taken at a local level.

Strategies like this, the cold-blooded killings of civilians, is not the sort of decision taken at a local level.

That would be a decision taken probably at the battle group level, if not higher. So in terms of command responsibility, the chain of responsibility, would go quite a considerable distance up the chain of the Sri Lankan army because these mass executions where we're dealing with perhaps hundreds of killings is just not the sort of thing a local commander would take upon himself. That would be something that would be sanctioned at a high level, particularly the killing of women, particularly the killing of children, and as your video indicates, the killing of a very well known Tamil woman, that's not something that a local commander would have taken upon himself.

President Mahinda Rajapakse (Reuters).

So further up the chain – the Chief of Staff, the Defence Secretary, even the Commander in Chief of the Armed Forces, President Rajapakse?

Quite possibly yes. There are problems bringing a president to trial because of head of state immunity but certainly everybody below that is capable I think on basis of this evidence of certainly being investigated. It provides a basis for investigating and the investigation stands a very good chance of revealing documents and orders sanctioning this killing.

What chance is there do you believe, given Sri Lanka's own apparent unwillingness to investigate its own army and its own commanders, what chance is there of bringing those responsible for these apparent war crimes to trial?

This is really something that has to be tackled on the international level, as in Yugoslavia, as in Sierra Leone, where a country is unwilling to investigate war crimes carried out within its own territory, it's the international community that's acted. And this is something that the United Nations has to take on. The UN has to set up an investigation, has to set up a tribunal. There have been several of these now as well as Yugoslavia and Sierra Leone, there’s been one for the Lebanon, so this is something that is becoming more commonplace and it really is something that the UN really has to take on given the scale of the war crime that we see here. The Geneva Conventions were passed to stop this sort of thing happening 60 years ago.

This is the sort of thing we had during the Second World War and the fact that mass killings of unarmed combatants and unarmed civilians is still going on in 2010 really isn't something that the UN can ignore or should ignore.

The fact that mass killings of unarmed combatants and unarmed civilians is still going on in 2010 really isn't something the UN can ignore or should ignore.

So the unwillingness of the Sri Lankans to cooperate is understandable given its their troops that have carried out these atrocities but there is a well recognised remedy and that is for the UN Security Council and the UN bodies to set up an investigation and to identify those responsible and to bring them to trial, under threat of sanctions to Sri Lanka if they won't co-operate.

These are crimes under UK law – the International Criminal Court Act 2001 makes grave breaches of the convention "try-able" in the UK and this is something therefore that the UK government and the UK prosecution authorities have got jurisdiction over and you've a dossier of evidence which has been put together in which individual commanders and individual soldiers could be identified then that would provide a basis for the Metropolitan Police War Crimes unit to start investigating and if necessary make extradition requests to Sri Lanka – if the Sri Lankans themselves won't investigate it. That's what the ICC Act is there for, if countries won't investigate themselves, and prosecute those responsible for war crimes, the UK is under an international law obligation to do so.

With evidence like this, which one only has very, very rarely, it really would be inexcusable if in the face of non action by the Sri Lankans, the UK authorities were to take no action.

We do not know how Isaipriya and the woman lying next to her died. On the Sri Lankan government website they claim that she was killed in battle, but we see her with her hands apparently bound and a sheet covering her body. The next image we see on the video is her with a gash on her face in a field with lots of other bodies, who we know either are being executed or look as if they have been executed. But we don’t know how she died.

There isn't the obvious head injuries as there are with the male victims, who have obviously been shot in cold blood. To my eye, two things stand out –one is the fastening of the hands behind the back and it's difficult to see how that could have happened if this death occurred in the course of battle and secondly there's the absence of any weapons.

And thirdly the bodies look posed or arranged they don't look like they've fallen necessarily in battle as the result of a battle-led injury so it's difficult to think of a mechanism how they could have died other than a cold blooded execution. And certainly the still photograph we looked at, their backs are to some sort of culvert, so if they were shot from – if they've fallen there – and been shot from the front. One would have expected to see entry wounds at the front. The fact that their backs are to sandbags or some sort of culvert or trench does rather make it look like they've been shot there and their bodies arranged, and I would expect that there are probably injuries to the back and they've been shot in the back.

Read the Channel 4 News special report on the civil war in Sri Lanka

What about if she was injured in battle?

If they've been injured in battle and left to die with their hands tied up, it's still a grave breach. Grave breach is not just killing of civilians, it's ill treatment, and I think most people would agree that failing to treat somebody who is injured, tying her hands behind her back and leaving her to die in the bottom of a trench is ill treatment of an unarmed civilian or an unarmed combatant.

Isaipriya

Even if you don't see the wire or rope tying her hands together?

It would be very hard to see. There are two people shown on the still they've both got their hands in the same position, that's certainly pretty powerful circumstantial evidence that their hands are tied even though you can't actually see the rope. The other point I would make about that is that it just doesn't have the feel of battle. The sounds that are going on around and about, while there is fighting or sounds like fighting somewhere in the distance, there doesn't appear to be the marks of battle.

I'm quite sure these mopping up operations did involve the mass killings of civilians or combatants who were trying to surrender. Mopping up operations is just really a euphemism.

There don't appear to be shells, there don't appear to be tanks, it just doesn't have the feel of a battlefield scenario and given that footage is interspersed and those stills are interspersed with undeniable footage of executions, again that lends support for the fact that these aren't deaths of battle. One doesn’t have a whole film of deaths in battle and a whole film of executions – this is interspersed with execution scenes which again from a forensic point of view supports the notion these women were killed and didn't die in battle.

While the Sri Lankan government may be denying the authenticity or maybe giving a different version of events, the way to solve what happened is for the Sri Lankan government to allow an independent investigation and then if the Sri Lankan government's version of events is true, that's what the investigation will reveal. If the Sri Lankan government is declining to allow an investigation again that's pretty powerful evidence that their version of events is not true and they don't want the truth to come out.

You're aware the Secretary General of the UN has set up a panel to investigate whether or not there should be an inquiry and the deadline for submissions to this inquiryis 16 December. So we have forwarded the video to them already and now we're forwarding this new evidence to them that we have about Isaipriya.

On the question of whether there should be an inquiry, this is astonishingly powerful evidence of a type I've only seen in a handful of times – there's some footage from Yugoslavia about mass killings – and this is up there. It's within a very, very rare category of evidence where killings are actually captured on tape and the idea that there can be a debate about whether there should be an investigation in the face of evidence like this is very surprising. So this evidence should lead to only one conclusion which plainly is there needs to be a full investigation and there needs to be prosecution of the people responsible.

We know that there were "mopping up" operations going on, on the 18 May, trying to hunt down at the time the Tamil Tiger leader, who they didn't find until the 19, the next day.

Certainly but "mopping up" operations - there's a very fine line between genuine military operations and just killing unarmed pockets of soldiers who may be trying to surrender. And certainly the Tamils that one sees before their deaths they don't look the most resilient of fighting troops and I'm quite sure these mopping up operations did involve the mass killings of civilians or combatants who were trying to surrender. Mopping up operations is just really a euphemism.