By Robert Evans

GENEVA, June 4 (Reuters) - A United Nations investigator called on Wednesday for the creation of a special inquiry into the 2003 killing in Baghdad of 22 U.N. staff, including then High Commissioner for Human Rights Sergio Vieira de Mello.

Leandro Despouy, specialist sleuth for the U.N. Human Rights Council on the independence of judges, said the failure by Iraq or United States occupation forces to establish the truth about the bloodshed undermined the credibility of the world body.

"I propose the establishment of a commission of inquiry composed of eminent experts to finally bring to the light of day exactly what happened and who was responsible," Despouy, an Argentine lawyer, told a news conference.

Vieira de Mello, a Brazilian widely tipped as a future U.N. secretary-general, died with his colleagues when suicide bombers exploded a truck outside the U.N. compound in the Iraqi capital on Aug. 19, 2003, five months after the U.S.-led invasion.

A long-time U.N. official, Vieira de Mello had been seconded from Geneva by then Secretary-General Kofi Annan as the world body's chief representative supervising relief operations and maintaining links with the occupation authorities.

But mystery still shrouds the event, which led the United Nations to pull all staffers out of Iraq.

Several efforts to pin down how the bombing happened and who organised it have failed to come up with any fully credible explanation, said Despouy. This had left the impression that such crimes against the United Nations could be committed with impunity.

The investigator, who also set out his views in a report to the 47-member Council, said he remained mystified over why the Iraqi government had not halted the execution last July of a man who may have had information on the attack.

He said he had made several pleas to Iraqi leaders and to senior U.S. and British officials for a suspension of the sentence -- for another crime -- on the man, who had confessed to having taken part in preparing the bombing.

"He was the last survivor of the people presumed to be behind the attack," declared Despouy, "and perhaps could have given invaluable testimony."

The appeals had been addressed earlier in 2007 to leaders of Western governments who had voiced admiration for Vieira de Mello, including the then British prime minister Tony Blair. He did not know if they had taken any action.

"The gravity of this event, and the huge importance it has both for the families of the victims who want to know how their loved-ones died and for the credibility of the United Nations, should not be ignored," he told the news briefing. (Editing by Janet Lawrence)

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