In March 2009, in a small town in the north of the island, hundreds of Sri Lankan Tamils had formed a queue to collect kanji, a boiled rice-porridge dish, which was being distributed by a charity later found to have ties with armed groups.
Then the explosions began.
"I remember that one shell fell adjacent to the food distribution line, and when it exploded the shrapnel hit many civilians who were standing in line," said one eyewitness.
"I remember seeing many dead bodies... A number of these dead bodies were elderly people and children."
At the time, the north was the scene of the concluding stages of a 26-year civil war fought between the Liberation Tigers of Tamil Eelam (LTTE), an armed separatist organisation known as the Tamil Tigers, and government troops - a fight which the UN says may have claimed as many as 40,000 civilian lives in its final months.
|Sri Lanka war report 'shows rights abuses'
Speaking to researchers from the Public Interest Advocacy Centre, an Australian law policy group, the unnamed witness said that the artillery shells that hit the food distribution centre had come from the west side of the coastal strip - where the Sri Lankan security forces were based.
This is one of numerous witness accounts documented by a team of international lawyers, which reported military attacks on hospitals and humanitarian sites.
The centre's International Crimes Evidence Project (ICEP) this week published its findings in a report titled: Island of Impunity? It detailed evidence that the government may have covered up human rights violations, and systematically cleared mass graves - destroying evidence of killings on a huge scale from dozens of sites within the "safe zone" between January and May 2009.
Both sides are responsible for atrocities committed during the war, the report notes, but it blames government troops for carrying out indiscriminate assaults on known civilian areas, or "No Fire Zones", between September 2008 and May 2009.
"The attacks analysed in this report reveal an intensified practice of indiscriminate artillery area bombardment," read the report. "These attacks killed and wounded scores of civilians who were attempting to take shelter from the sustained shelling, or trying to perform basic activities like collecting food or accessing medical treatment."
The report documents recurring instances of the LTTE committing war crimes, including forcibly preventing civilians from escaping the conflict zone, and also alleged post-war killings where Sri Lankan Special Forces were reported to have unlawfully killed both prisoners and civilians in captivity, including Balachandra Prabhakaran, the 12-year-old son of LTTE leader Vellupillai Prabhakaran.
Although international rights organisations and media were not allowed into the country during the last months of the war, a documentary named Sri Lanka's Killing Fields - aired by Britain's Channel 4 in 2011 - compiled a series of amateur videos, which appear to support some of these allegations.
Vellupillai's death on May 19, 2009, brought an immediate end to the war.
Paikiasothy Saravanamuttu, the executive director of Colombo-based advocacy group Centre of Policy Alternatives, told Al Jazeera that the strength of the report lies in its prosecutorial focus. It will be used to garner support for international investigations ahead of the 25th UN Human Rights Council sessions in Geneva next month - at which the United States delegation will head a third resolution pressing the island nation to investigate the conduct of its troops.
"In the absence of substantive progress on the accountability question, the report reinforces the call for an international investigation. It remains to be seen as to whether the current council will have the stomach for this. It should be noted too that any UNHRC resolution is not mandatory," said Saravanamuttu.
|Sri Lankan MP responds to PIAC report
"It is clear that the issue will not go away, and as long as it stays on the international agenda there is every likelihood of more information coming to light - further underscoring the case for an investigation," he added.
The Sri Lankan government, however, vehemently denies the allegations brought against them. MP Rajiva Wijesinha, a presidential adviser on national reconciliation, told Al Jazeera that his government "will not have international interference".
The government, he said, would look into conducting its own investigation into the allegations through its Lessons Learnt and Reconciliation Commission (LLRC), and emphasised that reconciliation "is not about vindictiveness", but "working toward a common future".
Gordon Weiss, who Wijesinha called "a liar", is one of the authors of the ICEP report, and also authored a controversial book, titled The Cage: The fight for Sri Lanka and the last days of the Tamil Tigers.
"The alleged destruction of the mass graves by the government of Sri Lanka is really quite disturbing because it flies in the face of government claims that it intends to bring about reconciliation between the two groups," said Weiss, who said that the destruction of these graves and evidence was a "heinous crime".
"There is very strong evidence that there was, at worst, purely intentional… and reckless shelling of these areas with really no heed given to the preservation of civilian life," Weiss, a former UN spokesperson for Sri Lanka, told Al Jazeera.
Graves are still being uncovered in the north of the country, most recently on December 20, 2013. Construction workers were laying water pipes in the northeastern district of Mannar when they discovered a 400 square metre mass grave, revealing at least 53 unidentified skeletal bodies.
The ICEP report's authors also include John Ralston, who led UN inquiries into Yugoslavia and Darfur, and Desmond Travers, co-author of the Goldstone Report on Gaza.
Rejection and denials
Navi Pillay, the UN High Commissioner for Human Rights, visited Sri Lanka in August 2013 - and said that the country was developing authoritarian tendencies.
The UN rights chief had called for a war crimes investigation into "credible allegations" that up to 40,000 civilians were killed during the final stages of the civil war.
Again, Colombo lashed out at the comments, rejecting an international inquiry.
|Sri Lanka's UN ambassador comments on PIAC report
"The High Commissioner's observation... is a political statement on her part, which clearly transgresses her mandate and the basic norms which should be observed by a discerning international civil servant," the government's Information Department said in a statement last September.
"The judgement on the leadership of the country is better left for the people of Sri Lanka to decide than being caricatured by external entities influenced by vested interests."
In November, in comments echoed by Washington, British Prime Minister David Cameron set a deadline of March 2014 for President Mahinda Rajapaksa's administration to investigate allegations of war crimes, or face an internationally backed UN probe.
But the Sri Lankan president defiantly retorted that the government would "take its own time".
With the international spotlight again turning towards Sri Lanka, Palitha Kohona, Sri Lanka's ambassador to the UN, said this week that the most recent report did not substantiate any charges made against his country's government.
"One can go on making these allegations endlessly. And the answer to that is not an international investigation, but giving time and space to the Sri Lankan government to deal with these issues on its own," Kohona told Al Jazeera.
He added that the US resolution at Geneva next month would not serve any domestic purpose, and would act as an obstacle to the country's reconciliation process.
On Saturday, US Assistant Secretary of State Nisha Biswal told reporters in Colombo that the US government was frustrated with "insufficient progress" in addressing justice, reconciliation and accountability, four years after the end of the war.
Sri Lanka's government has repeatedly denied the allegations.
In a televised address on Tuesday, President Rajapaksa said that claims of war crimes were not founded on "peace, fair play or justice".
|Sri Lanka's President Mahinda Rajapaksa [Reuters]
"We have acted to provide justice through the law by defeating terrorism," he said.
Wijesinha told Al Jazeera that an international investigation must be avoided, as "it's a betrayal of the basic principle of the United Nations - which is national sovereignty - and secondly, we know perfectly well that these things are not objective".
While the government remains defiant, the president insists that such an inquiry would be aimed at pleasing the large Tamil diaspora living in Western countries.
Colombo has previously criticised the Tamil diaspora for funding the LTTE, and considers any international attempts to revive the rebel group a threat to national security. However, while many among the diaspora support the notion of Tamil Eelam , an independent state within the scenic island, analysts say there remains little popular support for another armed struggle.
With more than 100,000 people killed in Asia's longest civil war in the modern era, and the focus on the conflict at the forthcoming UNHRC summit in Geneva, an international inquiry seems imminent - and diplomats and the diaspora alike are waiting to see what Colombo does next.
Follow Rahul Radhakrishnan on Twitter: @RahulRadhakris