Italian journalist Giuliana Sgrena was mistakenly shot by US forces just hours after being freed by her captors in Iraq.
The shots fired at her convoy wounded her and killed Nicola Calipari, an Italian secret service agent.
Italian Prime Minister Silvio Berlusconi, immediately summoned the American ambassador, demanding explanations and declaring someone had to take responsibility.
Berlusconi said he personally knew Calipari who had worked on previous hostage release cases in Iraq.
Ms Sgrena was abducted in Baghdad on 4 February.