The Pentagon's statement comes almost two years after America officially declared the war in Iraq over on 1 May 2003.
An internal Pentagon memo printed in The Washington Times stated: "This is war. About 500 American service members have been killed by hostile fire while operating on Iraqi streets and highways.
"The journalist was driving in pitch-dark and at a high speed and failed, according to the first reports, to respond to numerous warnings.
"Besides, there is no indication that the Italian security forces made prior arrangements to facilitate the transition to the airport."
The memo said Italian security forces failed to make arrangements for the safe passage of newspaper Il Manifesto's Sgrena, who was wounded when US troops opened fire as she was being driven to the airport in Baghdad after being released by her kidnappers, the newspaper said on Tuesday.
Friday's incident - in which Italy's top intelligence officer in Iraq, Calipari, was killed - has strained relations between the United States and Italy and is under a full scale investigation by the US State Department.
The US military has said its forces gave ample warning to the driver of Sgrena's car, which they said was fast approaching when they opened fire.
Sgrena has said they were not travelling fast and that she was deliberately targeted for murder because the US opposed negotiations with her kidnappers, a charge a State Department spokesman rejected.
"It's absurd to make any such suggestion that our men and women in uniform would deliberately target innocent civilians," said White House spokesman Scott McClellan, adding: "We regret this incident. We are going to fully investigate what exactly occurred."
"They told me: 'Now you are free to leave. But you must be cautious, because the US forces and the Iraqi police might not be willing to let you leave Iraq alive'
Italian journalist Giuliana Sgrena
Sgrena told Aljazeera that her captors had warned her that US forces would never allow her to leave Iraq alive.
She said: "They told me: 'Now you are free to leave. But you must be cautious, because the US forces and the Iraqi police might not be willing to let you leave Iraq alive'.
"They told me they would do their best to secure a safe departure for me but warned that the US forces might not permit this. This is exactly what they have said.
"I thought that was some sort of an anti-American propaganda before I was freed. But when the shooting occurred, I recalled what they had told me before my release.
"That was an interesting paradox, what has happened was exactly as they had told me."
Robert Maginnis, a retired army officer and military analyst consulted by The Washington Times, said the Italians should have given serious attention to their movements in Iraq.
"It seems to me that the Italian secret service considers this a James Bond movie in Baghdad," Maginnis said.
"They're driving around at night picking up a journalist who has been kidnapped and pretending they can get through a phalanx of checkpoints along the deadliest road in all of Iraq without being detected, much less shot up."
A US officer said Calipari acted
like a James Bond character
The incident has put a spotlight on friendly fire episodes that occur with some regularity in Iraq when motorists fail to heed warnings to stop at roadside checkpoints and are fired on by American troops who fear that the vehicle might be a weapon.
Cars and trucks are a common weapon in bombings and drive-by shootings.
A US official said from all the cars that passed through the checkpoint that night, the reporter's vehicle was the only one fired upon.
"Something that car did, caused the soldiers to fire," said the official, who asked not to be named.
The soldiers say they did not know that Sgrena and Italian agents were headed in their direction on the way to the airport for a flight back to Italy.