UN Secretary-General Mr Kofi Annan today called for the prosecution of those behind the "cold-blooded murder" of 22 people at the United Nations office in Baghdad one year ago.
Ceremonies marking the worst attack on a UN operation were held in Geneva, Baghdad and New York.
UN Secretary-General Mr Kofi Annan
Voicing concern that the world body had become "one of the main targets of political violence", Mr Annan vowed to protect UN aid workers from more tragedies.
About 150 people were injured, some permanently, in the truck bomb attack, which led to the withdrawal of UN international staff. Washington linked the bombing to al-Qaeda.
"Despite active efforts on our part, and despite the ongoing investigation by the United States, we are still waiting for answers," Mr Annan told an hour-long ceremony in Geneva attended by hundreds of staff and about 80 survivors and family members.
"However long it takes, I pray that the perpetrators are held to account and do not get away with this cold-blooded murder."
Mr Sergio Vieira de Mello, the UN High Commissioner for Human Rights who headed the UN mission in Iraq during the US-led occupation, was killed along with top aides.
Mr Vieira de Mello's mother, his French widow and their sons attended the ceremony during which Mr Annan unveiled a bronze plaque engraved with the victims' names.
A UN-produced video showed staff before the fateful day. "These are the 22 faces of our conscience, of our common humanity. They had no patience or apathy in the face of misery," the voiceover said.