The 52-year-old man was found dead Friday at a coalition prisoner of war camp near Nasiriyah, military officials said. He had been held since his May 3 capture, U.S. Central Command said in a statement.
The man was not one of the 29 members held by American forces of the 55 most wanted Iraqis, officials said.
Central Command officials declined comment on the cause of the prisoner's death. The death is being investigated by the Naval Criminal Investigative Service, suggesting there were indications the prisoner was killed.
The 1st Marine Division runs the POW camp where the death occurred, military officials said. The United States is holding more than 2,000 Iraqi prisoners of the thousands captured during and after the war. Hundreds of other Iraqis have been released.
The investigation is the first announced U.S. probe involving the death of an Iraqi prisoner of war. Findings of criminal wrongdoing could result in court-martial proceedings against those involved.
Pentagon officials said Wednesday they could not speculate on what criminal violations could be involved. Mistreating prisoners of war could also violate the Geneva Convention and other international law.
U.S. military investigators have launched a war crimes probe of Iraqi actions during the war, including evidence suggesting that some U.S. troops were killed after being taken prisoner by Iraqi forces.
The British defense ministry is investigating the deaths of two other Iraqi POWs and two other cases of alleged beatings or torture of prisoners under British control.
The Pentagon also is investigating the December deaths of two prisoners at a U.S. military base in Afghanistan. Military coroners ruled the two deaths were homicides, finding that both men had been beaten and one had a blood clot in his lungs.