Helton was meeting Sergio Vieira de Mello, the U.N. chief of mission for Iraq, who also died in the blast, sources said. The two were among at least 20 killed when a truck bomb blew up outside the building.
Rick Hooper, an American working for the U.N. Department of Political Affairs, was also among the dead, the United Nations said Tuesday. Hooper, a New Yorker, had worked with the United Nations peace coordinator in Gaza before coming to Baghdad.
Helton had been director of peace and conflict studies at the Council on Foreign Relations, a prestigious U.S. foreign policy think-tank based in New York. He had long studied and written about the plight of refugees around the world.