An Iraqi soldier has opened fire on American troops, killing two and wounding three others, US and Iraqi officials have said.
Iraqi soldiers are meant to provide the backbone of future security
The incident happened during a joint patrol in the north on 26 December, but fuller details have only now emerged.
An Iraqi general said the patrol had come under fire from gunmen in the city of Mosul, but his soldier had "abused" the situation and shot the Americans.
It is thought to be the first such case since the US-led invasion of 2003.
The US initially reported the deaths of Capt Rowdy Inman, 38, and Sgt Benjamin Portell, 27, as resulting from hostile small arms fire.
But Iraqi general Mutaa al-Khazraji said: "They [the joint patrol] were attacked by gunmen and the [Iraqi] soldier abused the situation and killed the two soldiers."
He said the Iraqi serviceman had been "an insurgent infiltrator". He was arrested and is being questioned.
An interpreter was also wounded alongside the three other soldiers.
Tensions remain high in Mosul - an ethnically mixed area that is home to a variety of Muslim and Christian communities.