Now, two years on, an Australian citizen and senior Sri Lankan diplomat stands accused, in an application to the International Criminal Court, of complicity in the murder of surrendering Tamils. A Herald investigation examines one desperate final act in Sri Lanka's civil war on a lonely, bloody beach at Mullaitivu.

The ruthless separatist war waged by the Liberation Tigers of Tamil Eelam against the Sinhalese-majority Sri Lankan government came to its bloody end, after more than a quarter of a century, in May 2009.

The terrorist army, which at one stage controlled a third of the island, was reduced in the end to a narrow strip of coastal land, hemmed in by the sea, a lagoon, and a relentless assault from the advancing army.

Many Tigers vowed to fight to the death rather than lay down their arms, but as the bodies piled up on the beach, some sought a way to surrender, for themselves, and for the thousands of civilians they held hostage.

Under constant heavy shelling, on mobile phones with waning batteries, they rang and sent text messages to anyone they thought could help rescue them.

They contacted diplomats, journalists, workers of non-governmental organisations, seeking anyone who could broker an agreement between their rump of a resistance and the government.

They made dozens of calls.

Through a European intermediary, they got a message to Dr Palitha Kohona, Sri Lanka's foreign secretary, and at that time the public face of Sri Lanka's war against the Tigers.

Dr Kohona is dual Sri-Lankan Australian citizen, a former Australian diplomat and trade negotiator for the Department of Foreign Affairs and Trade. He is now an ambassador for Sri Lanka, the country's permanent representative to the United Nations.

He was contacted with a message on behalf of a man called Pulidevan, who was the head of the Tiger peace secretariat. Pulidevan was not a soldier, but a key Tiger policymaker. He offered an unconditional surrender.

Willing to walk out with him were members of his family, Nadesan, the head of the Tigers' political wing, a Tigers' Colonel called Ramesh, and more than a dozen civilians.

Over several hours it was negotiated that those who wished to could safely surrender under a white flag. They were told a route to walk from behind a defensive bund into the custody of the army's 58th division. The troops were told to expect them.

At 8.46am on Sunday, May 17, Dr Kohona sent a text message to a third party intermediary, which was passed on to Pulidevan.

"Just walk across to the troops, slowly! With a white flag and comply with instructions carefully. The soldiers are nervous about suicide bombers."

At 6.30am the next day, Pulidevan and Nadesan, each carrying a white flag above his head, and followed by a group of about 15 that included their families, walked from their hideout towards the 58th division.

A Tamil man who was in the war zone and watched them leave - who has since fled Sri Lanka, but asked that his name be withheld for fear of reprisals - told the Herald they walked as instructed and none were armed.

"I saw them with my own eyes. Pulidevan and Nadesan and then behind them the other people. I saw them walk towards the soldiers. The soldiers had trucks, they took the people behind the trucks and behind the sand mound, we couldn't see them any more. Then we could just hear shooting, and people yelling. The shooting was fast, like a machinegun.''

Reports at the time said the wife of Nadesan's wife, a surrendering Tamil, called out to the troops in their native Sinhalese: "He is trying to surrender and you are shooting him."

The deaths of Pulidevan and Nadesan were reported within half an hour. The body of Pulidevan's wife was found, too, also shot dead.

The war ended that day. The bodies of some of those who surrendered were found in the days following, but many were not. It is believed that none of those who surrendered survived.

Dr Kohona told the Herald the text message was not part of any negotiations with the Tigers. "I have said … categorically that as foreign secretary of Sri Lanka, I never had the authority to issue orders to troops or to discuss surrender terms of any terrorists, either directly or indirectly."

He did not "honestly believe" an effort was made by the group to surrender.

"The text is likely to have been in response to an inquiry, but not from anyone associated with the LTTE. This was not an effort to arrange a surrender, which I had no authority to do.

"In my understanding this is how surrenders normally take place. This was all the advice I could give."

The Herald has confirmed with three independent sources the message sent by Dr Kohona to a third party was in response to an offer from Pulidevan of a surrender. The message was passed to Pulidevan.

But while Palitha Kohona was regarded as a key player in negotiations, evidence is emerging the Tigers' planned surrender was foreknown throughout the Sri Lankan government and military, to the very highest levels.

The report to the UN Secretary-General on the conduct of the war, released last month, found: "both [Sri Lankan] President [Mahinda] Rajapaksa and [his brother] Defence Secretary Gotabaya Rajapaksa provided assurances that their surrender would be accepted.

"These were conveyed by intermediaries to the LTTE leaders, who were advised to raise a white flag and walk slowly towards the army, following a particular route advised by [Gotabaya] Rajapaksa."

Leaked US embassy cables show Norway's ambassador was in regular contact with Tamil Tiger leaders and with Gotabaya Rajapaksa, urging the government to accept a surrender.

And a deposition from a senior army officer, seen by the Herald, says there were instructions from a field commander "to get rid of the LTTE cadres who are surrendering, without adhering to the normal procedures".

The Sri Lankan government has maintained the surrendering Tamils were shot by other cadres as they deserted. It says it pursued a humanitarian rescue operation.

Dr Kohona himself addressed the UN Security Council this week, telling it that at the end of the war, "the government adopted a zero civilian casualty policy''.

He told the Herald he has "absolutely no idea" how Pulidevan and Nadesan died.

"The version [presented by the report to the UN] appears to be a convenient entree to establish criminal conduct. The rump LTTE has been working overtime clutching at straws to get even.''

The UN Secretary-General's report described numerous contested incidents in the final stages of the war. It concluded it was "unable to accept the version of events held by the government of Sri Lanka".

Corrections:

An earlier version of this story wrongly stated that Sri Lanka's Defence Secretary is Basil Rajapaksa. The position is held by Mr Rajapaksa's brother, Gotabaya.

An earlier version of this story mistakenly attributed the quote "He is trying to surrender and you are shooting him" to the wife of Pulidevan. The quote was by the wife of Nadesan.

An earlier version of this story said: The UN Secretary-General's report found it was "unable to accept the version of events held by the government of Sri Lanka".

Press Council adjudication:

This article was the subject of an Australian Press Council adjudication.