The US military is to investigate the circumstances in which a male relative of Iraq's envoy to the United Nations died during a search of his home.
US troops have been accused of heavy-handedness in Iraq
Samir Sumaidaie has accused US soldiers of killing his 21-year-old cousin Mohammed in "cold blood".
His cousin, he said, died from a bullet wound after letting US troops into his house, near Haditha in western Iraq.
The US confirmed Mohammed had been "killed during a search of his family's home" in June and promised an inquiry.
A statement said the death had been referred to the Naval Criminal Investigative Service (NCIS) after a preliminary investigation.
The NCIS can investigative suspected crimes by people working under the Department of Navy, which includes the US marines.
Iraq's ambassador to the UN last month demanded an inquiry into his relative's death.
A letter written by Mr Sumaidaie to his colleagues said his unarmed cousin had been assisting marines in the search of his house when he was killed.
Mr Sumaidaie said the ramifications of such a "serious crime" were enormous for both the US and Iraq.
He said Mohammed, an engineering student, was visiting his family home when some 10 marines with an Egyptian interpreter knocked on the door at 1000 local time.
He opened the door to them and was "happy to exercise some of his English", said the ambassador.
When asked if there were any weapons in the house, Mohammed took the marines to a room where there was a rifle with no live ammunition.
It was allegedly the last time the family saw him alive. Shortly after, another brother was dragged out and beaten and the family was ordered to wait outside.
As the marines left "smiling at each other" an hour later, the interpreter told the mother they had killed Mohammed, said Mr Sumaidaie.
"In the bedroom, Mohammed was found dead and lying in a clotted pool of his blood - a single bullet had penetrated his neck," the Iraqi envoy said.