During the past year, the U.N. has launched independent investigations into possible war crimes in Ivory Coast, Libya and Syria.

But in Sri Lanka, where as many as 40,000 civilians were killed in the final phases of the country's civil war, the U.N. has been unable to muster support for an independent investigation into atrocities.

So why has Sri Lanka been different? The scale of killing -- perhaps the worst spasm of violence committed by any government under U.N. Secretary General Ban Ki-moon's and President Barack Obama's watch -- far surpasses the death toll from government crackdowns during the Arab Spring. And a U.N. panel established by Ban to assess Sri Lanka's commitment to hold perpetrators of mass crimes accountable has concluded that the government has failed after three years to demonstrate that it is prepared to conduct a credible accounting.

The panel's findings, which were released last April, constituted a devastating indictment of the country's military conduct during the final stage of the 28-year war. The panel accused government forces of shelling hospitals, no-fire zones and U.N. facilities, and blocking the delivery of humanitarian aid to victims of the war. It also accused the separatist Tamil Tigers of committing gross human rights abuses, particularly by using civilians as human shields. The three-member panel recommended Ban set up an "independent international mechanism" to probe "credible" allegations of crimes against humanity by both sides in the conflict.

But the U.N. Security Council has shown little inclination over the past year to confront Sri Lanka over its conduct. And neither Ban nor the United States have thrown their prestige behind a plan to establish a commission of inquiry.

Instead, the international community has looked to the Sri Lankan government to do the job itself. The U.N. has awaited the conclusion of a Sri Lankan inquiry by the so-called Lessons Learned and Reconciliation Commission before rendering a judgment over the government's commitment to pursue alleged war criminals.

The commission recently concluded its work, producing a final report that garnered some mild praise from the U.N. and the United States, but failed to convince either that its efforts constituted a credible effort to hold perpetrators accountable for their crimes. "The report falls short on addressing allegations of violations of international humanitarian law by both sides in the conflict," Secretary of State Hillary Clinton recently wrote in a letter to Sri Lankan Foreign Minister G.L. Peiris.

The Sri Lankan government launched an all-out offensive in 2008 in an effort to crush the Tamil Tigers, one of the world's most violent and ruthless insurgencies. The operation, which centered on a Tamil stronghold in the Vanni region of Sri Lanka, succeeded in wiping out the armed movement in May 2009. But the operation took a devastating toll on ethnic Tamil civilians, who were largely trapped between the rival forces.

Shortly after the conflict ended, Sri Lankan President Mahinda Rajapaksa, facing a push by the U.N. Human Rights Commissioner to establish an international commission of inquiry, persuaded Ban to strike an agreement allowing Sri Lanka's authorities to do the job. Sri Lanka established the Lessons Learned and Reconciliation Commission to examine conduct by both sides in the final stage of the 28-year-old civil war.

Steven Ratner, one of three members of a panel established by Ban to assess the Sri Lankan commission's progress, spoke on behalf of the panel earlier this month when he told diplomats and human rights advocates in New York that the commission had taken some positive steps, including its affirmation of the government's responsibility for prosecuting perpetrators of the worst crimes. But he said it failed to make "bona fide" effort to ensure perpetrators are held accountable, according to an official present at the meeting.

The U.S. ambassador to the U.N. Human Rights Council, Eileen Donahoe, echoed some of the panel's findings, announcing today that she would press for the adoption of a resolution in the Human Rights Council that will call on Sri Lanka to undertake a more "credible accountability" effort.

"We believe there cannot be impunity for large-scale civilian casualties, and that if there is to be real reconciliation it must be based on an accounting of the truth," she said. "We are saying that you have to show your citizens that you will not just be taking this report and putting it in a drawer."

But Donahoe also made it clear that Sri Lanka would bear the responsibility.

Donahoe said that the effort to rally support for action in Sri Lanka has been more difficult than in places like Libya and Syria because the atrocities occurred years ago.

"We could have, of course, followed the proposal of the NGOs and others to ask for a [international] commission of inquiry, basically say that the domestic efforts had not been valuable," she said, "There was a question of the likelihood of success...We have to be pragmatic. What can we do?"

The U.S. action was welcomed by human rights activists, who said it marked the first time the Human Rights Council had demanded Sri Lanka take substantive action. Nearly three years ago, the Geneva-based rights body rejected a proposal by U.N. High Commissioner for Human Rights Navi Pillay to set up a commission of inquiry, and instead adopted a resolution commending Sri Lanka for its victory over the Tamil Tigers.

James Ross, the legal and policy director for Human Rights Watch, said that while his organization favors the establishment of an independent commission of inquiry for Sri Lanka, he sees the U.S. move as the beginning of a process that holds the promise of bringing about some degree of justice for victims.

Follow me on Twitter @columlynch.