Italian newspapers on Thursday said the probe has been held up due to reluctance by Italian investigators to accept the US version of the shooting which occurred while Italian secret service agents were driving freed Italian hostage Giuliana Sgrena to Baghdad airport.
In the United States, however, NBC News said a preliminary report from the US-Italian commission had been completed and cleared the US troops of any wrongdoing in the killing of Nicola Calipari.
But an Italian Foreign Ministry official disputed the NBC account, saying the commission was continuing its work. The official, speaking on condition of anonymity, declined to comment on the reports of a clash among the investigators.
The US State Department denied the investigation had absolved the Americans of responsibility.
Calipari was killed while escorting
Giuliana Sgrena (C) to the airport
"The investigation is still ongoing," spokesman Thomas Casey said. "Anyone claiming that conclusions have been reached, and they know what they are, must be misinformed".
The commission, ordered by Washington, includes two Italian members and is led by a US brigadier-general. It is expected to release its findings by mid-April.
Italian newspapers Corriere della Sera and La Stampa said a final conclusion by the commission is being delayed by the reluctance of the Italian members to accept all the aspects of the US version of events.
According to the Italian papers, a point of contention is US authorities' refusal to allow Italian investigators to examine the car in which Calipari was travelling when he was shot.
Calipari was killed on 4 March by a US patrol as he escorted Sgrena to the Baghdad airport.
Another intelligence agent and Sgrena were wounded in the shooting, which outraged Italy.
Italy has denied a US claim that the car was speeding and refused to stop following warnings from the US patrol.
NBC said that the joint inquiry failed to resolve the dispute over the speed of the car, with the Americans claiming it approached the patrol at about 80kph and the Italians saying it was travelling much slower.