Several U.S. agencies are investigating a widening network of criminal cases involving the purchase and delivery of billions of dollars of weapons, supplies and other materiel to Iraqi and American forces, according to American officials. The officials said it amounted to the largest ring of fraud and kickbacks uncovered in the conflict here. The inquiry has already led to several indictments of Americans, with more expected, the officials said. This article is based on interviews with more than a dozen federal investigators, congressional, law enforcement and military officials, and specialists in contracting and logistics in Iraq and Washington, who have...