Italian secret agent Nicola Calipari was killed when US troops opened fire as he escorted freed journalist Giuliana Sgrena to Baghdad airport.
He had just helped to secure her release after she had spent more than a month as a hostage in Iraq.
The US, the Italian government and Ms Sgrena have differing accounts of what happened, while prosecutors in Rome are still conducting their own investigation.
The US report found that the US troops who were manning the checkpoint had followed the rules of engagement and should not be sanctioned.
It also establishes that, 20 minutes before the shooting, a highly-placed Italian officer in Iraq confirmed to a US aide that the flurry of activity along the Baghdad airport road had something to do with the Italian journalist.
The Italian then told the American that it was best that no one should know and the latter interpreted this as an order not to divulge the information.
However, the US report goes on to note: "Prior co-ordination might have prevented this tragedy."
After spotting the car, which they thought was travelling "in excess of 50mph (80km/h) and faster than any other vehicles that evening", one of the troops shone a spotlight on it before it reached the checkpoint's "Alert Line".
After that, he shouted at the driver and then fired a few warning shots but the car did not slow down.
Eventually, he fired into the car's engine block "in an attempt to disable it" and stopped shooting "as he saw the car slow down and roll to a stop".
"Approximately four seconds had elapsed between the firing of the first round and the last round, and no more than seven seconds from the time the car crossed the Alert Line until it came to a stop," the report says.
At that point, Calipari was wounded but still alive. The soldiers tried to bandage him but he died minutes later.
The Italian intelligence agent who was driving the car reportedly told the soldiers he had heard shots from somewhere "and that he panicked and started speeding, trying to reach the airport as quickly as possible".
A forensic examination of the car found 11 bullet holes and a trajectory analysis showed they had all come from one point of origin - implying that only one man had shot at the car.
But the document acknowledges that "due to it being their first full day on shift," the soldiers had "lacked experience in issuing operational orders and in battle tracking security forces during execution of blocking missions".
US MILITARY JUST AFTER THE KILLING
The US military said soon after the shooting that Calipari's car had been speeding as it approached a coalition checkpoint in western Baghdad at 2055 (1755 GMT).
Soldiers used "hand and arm signals, flashing white lights and firing warning shots" to get the driver to stop.
When the car did not stop, soldiers shot into its engine block.
US Lt Col Clifford Kent of the 3rd Infantry Division in Baghdad said the checkpoint where the shooting had happened was a temporary set-up.
According to the Associated Press news agency, when asked how easy it would be to see US troops at the checkpoint at night, he said:
"Depending on where it is, that could be difficult... but if you're seeing soldiers in military uniform with military equipment, if you know it's a dangerous area, then... you need to maintain your awareness.
"The event was very tragic, and my condolences go out to those killed and injured."
ITALIAN GOVERNMENT: OFFICIAL REPORT
Although the facts of the shooting may never be known, it appears that some of the US soldiers involved may have acted "instinctively" due to a combination of stress, inexperience and the tense circumstances on the night, Italy's official government report says.
The 52-page document published by the intelligence services queries the US military's procedures, but does not find evidence that soldiers deliberately sought to kill Calipari.
"It is likely that the state of tension stemming from the conditions of time, circumstances and place, as well as possibly some degree of inexperience and stress, might have led some soldiers to instinctive and little controlled reactions," the report says.
"On the other hand, the lack of formal references to clear rules that could have and should have been observed renders problematic precise pinpointing, attribution and weighting of specific individual responsibility."
The Italian report concurs with the US inquiry's finding about the Italian officer's warning to his US aide about the rescue operation.
While agreeing on many other points, the Italian version:
- emphasises the lack of warning signals - such as signs, bright cones, concertina wire - given to motorists of an impending roadblock
says the vehicle speed was half that reported by the Americans, denies there was any acceleration and describes the time between the warning shots and follow-up fire as "excessively short"
notes that the immediate removal of the vehicles involved, and the destruction of the US soldiers' duty logs, made "objective conclusions" impossible
reports that an Italian military commander was not allowed to visit the scene in the immediate aftermath while US officials were given access.
Rome prosecutors are conducting their own investigation into the shooting, involving the analysis of the car in which the Italians were travelling. Criminal charges could stem from their eventual findings.
ITALIAN GOVERNMENT: INITIAL COMMENTS
Italian Prime Minister Silvio Berlusconi noted soon after the killing that evidence given by the Italian survivors did not tally with US officials were saying.
He quoted the version of events given by another agent in the car with Calipari.
"A light was flashed at the vehicle from 10m away.
"The driver at this point stopped the car immediately and at the same time there was gunfire for about 10 or 15 seconds.
"A few shots reached the vehicle - one killed Mr Calipari and another bullet injured Ms Sgrena in the shoulder...
"This reconstruction... does not coincide totally with what has been communicated so far by the US authorities."
Italy had made all necessary contacts for safe passage, advising the US military at the airport as Ms Sgrena was en route, Mr Berlusconi said.
His statement contradicted preliminary accounts from the US military that it had had no knowledge of the rescue mission.
Mr Berlusconi said Italy, as an ally of the US, had "the duty to demand from them the utmost truth" about what had happened.
Giuliana Sgrena has suggested her car was deliberately targeted. She recalled the events in her newspaper, Il Manifesto:
"The driver twice called the embassy and Italy to say that we were heading towards the airport that I knew was heavily patrolled by US troops.
"They told me that we were less than a kilometre away... when... I only remember fire.
"At that point, a rain of fire and bullets hit us, silencing forever the cheerful voices of a few minutes earlier.
"The driver started yelling that we were Italians. 'We are Italians, we are Italians.'
"Nicola Calipari threw himself on me to protect me and immediately, I repeat, immediately I heard his last breath as he was dying on me.
"I must have felt physical pain. I didn't know why.
"But then it came to me in a flash, and my mind went immediately to the things the captors had told me.
"They declared that they felt fully committed to freeing me but I had to be careful: 'Out there are the Americans, who don't want you to go back.'
"Then, I had considered those words superfluous and ideological. But at that moment, they risked acquiring the flavour of the bitterest of truths. At this time I cannot tell you the rest."
"There was no bright light, no signal," she separately told Italian La 7 TV.
Ms Sgrena's editor, Gabriele Polo, said he was told by Italian officials that 300 to 400 rounds were fired at the car.
Corriere della Sera newspaper quoted the second Italian agent, who was driving, as saying the speed had been 40-50km/h.