A suicide bomber killed the head of the Iraqi police anti-corruption department in the northern city of Mosul on Sunday, officials said.
A US lieutenant-colonel said the bomber detonated explosives strapped to his body in the building where Brigadier Walid Kashmoula worked.
Earlier officials said it was a bomb attack.
Al Qaeda's wing in Iraq, led by Jordanian militant Abu Musab al-Zarqawi, said it had assassinated "the apostate Walid Kashmoula, who is the top American agent" in the area.
"This will be the fate of those who stand by the polytheists," al Qaeda Organisation for Holy War in Iraq said in a statement posted on an Islamist website.
Insurgents in Iraq have repeatedly targeted leading police and army officials.
Iraqi police and soldiers are also frequent targets of attack by gunmen and suicide bombers.
A roadside bomb killed four policemen yesterday in Kirkuk during a funeral for a colleague killed a day earlier.
