The military is investigating whether U.S. troops were responsible for the death of an Iraqi prisoner of war, officials said Wednesday.

The criminal investigation is the first involving the death of a prisoner in U.S. custody in Iraq. The British are investigating both the deaths of two Iraqis who were under British control and allegations of beatings or torture of prisoners by British troops.

U.S. authorities found the corpse of a 52-year-old prisoner Friday at a camp run by the 1st Marine Division (search) near Nasiriyah (search), officials said. The man had been held at the camp in southern Iraq since his capture May 3, U.S. Central Command (search) said in a statement.

Officials said the prisoner was not one of the 55 Iraqis most wanted by the Americans. More than half of the former Iraqi officials on that list have been captured and are being interrogated by American forces.

The Naval Criminal Investigative Service (search) is investigating the death, suggesting there is evidence the prisoner was killed.

Military officials would not say how the prisoner died. They refused to identify him or say whether he had been cooperating with American authorities. Officials also did not say whether the prisoner could have been attacked by other prisoners at the camp.

If U.S. troops were found to have killed a prisoner of war, it would be a significant black mark for the Pentagon, which has accused Iraqi forces of committing war crimes. Pentagon war crimes investigators are looking into whether Iraqis executed American troops after taking them prisoner during fierce fighting in Nasiriyah in late March.

Killing POWs is a violation of the Geneva Conventions (search) and other international law -- as well as the United States' Uniform Code of Military Justice (search).

Any findings of criminal wrongdoing in the Iraqi prisoner's death could result in court-martial proceedings against those involved.

American forces are holding more than 2,000 of the thousands of Iraqis captured during and after the U.S.-led war to topple Saddam Hussein. Hundreds of other Iraqis have been released.

At least one other Iraqi POW held by the Americans has been killed. A Marine shot an Iraqi prisoner to death March 29 in an incident that was ruled self-defense. That incident was not investigated as a crime.

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.