A report by French aid group Action Contra la Faim says Sri Lankan security forces killed 17 of its aid workers in 2006 and the charity is demanding an international inquiry into the massacre.

In a new report, ACF -- also known as Action Against Hunger -- claims that after carrying out its own investigation it is aware of who shot and killed the workers, mostly ethnic Tamil.

The workers were found lying face down, all shot in the head, in their offices in Muttur, about 15 miles south of the port city Trincomalee on the island's northeast coast.

The charity said in its report the aid workers "were likely assassinated by members of the Sri Lankan security forces and the criminals were covered up by Sri Lankan top authorities."

However, the report doesn't name names.

ACF said it had lost contact with its Muttur office about 7 a.m. Aug. 4 as the mostly Sinhalese security forces battled the rebel Tamil Tigers around the town.

Several days later, staff were found dead in the office area, "lined up inside the ACF compound, they had been shot dead at close range," the report said.

"Staying away from direct hostilities, ACF staff were not killed by stray bullets," the report said. "They were deliberately targeted, despite the fact that they were wearing white ACF T-shirts and weren't armed."

ACF said the report is based on information the charity obtained directly from "witnesses, diplomatic contacts and other sources close to the matter and [its investigation] offers several arguments in support of this allegation."

ACF has gone public with its report because of continuing frustration with Sri Lankan investigations that either have dragged on or concluded there was no evidence of security forces involvement, despite the charity's interviews to the contrary.

Internal Sri Lankan investigations are a "farce," ACF said.

Jean-Michel Grand, director of Action Against Hunger in the United Kingdom, told UPI the report was published after ACF "exhausted" all avenues of internal Sri Lankan investigations that failed to find the killers.

Investigations include a those by a magistrate court and the National Commission of Human Rights, both of which ran from 2006 to 2008.

Also inconclusive were a Presidential Commission of Inquiry from 2007 to 2010 and a two-year Lessons Learnt and Reconciliation Commission that ended last year.

"We don't know exactly the names of the perpetrators," Grand said. "But were looking at where the responsibility lies, the chain of command and who gave the order."

ACF had operated in Sri Lanka since 1997 consulting on water and irrigation projects as well as food security strategies. Grand said ACF pulled out in 2008 after frustrations with authorities over the inconclusive investigations and increasing intimidation by authorities of witnesses and its personnel.

Grand also said everything in the 28-page report is publicly available to the police.

What is needed now, Grand said, is an investigation by the U.N. Human Rights Council.

Human rights group, freedom of speech organizations and the United Nations have been critical over the years of how both sides in the conflict treated civilians.

Just before the war ended in 2009, U.N. High Commissioner for Human Rights Navi Pillay said the number of civilian deaths was "truly shocking" and could reach "catastrophic" levels.

As many as 100,000 people are thought to have been killed in the civil war.

Since war's end in 2009 tensions have remained high as a result of attempts to integrate Tamils into mainstream national life.

In February 2012, Sri Lanka's military appointed a five-member Court of Inquiry to look into civilian deaths allegedly at the hands of the army during the civil war.

The BBC reported a Sri Lankan military spokesman rejected ACF's allegations and said the charity is withholding evidence that could be used by investigators.

Sri Lankan military spokesman Brig. Ruwan Wanigasooriya said withholding alleged evidence instead of releasing it in support of the government's investigation raised suspicion about the group's motives.

"This is another instance of a pattern which has emerged since the end of the conflict where certain organizations level allegations against the government without providing sufficient details to enable an investigation," he said.