Meanwhile, Italian prosecutors conducting their own investigation on Thursday questioned former Italian hostage Giuliana Sgrena again in the Rome military hospital where she is recovering from a shoulder wound sustained in the 4 March shooting.
Official versions of the shooting from Rome and Washington, two long-standing allies, have differed on crucial points in the slaying of intelligence agent Nicola Calipari.
US army soldiers at a checkpoint near Baghdad airport opened fire at the car in which Sgrena, Calipari and another military secret services official were travelling, less than an hour after the journalist was released by her captors.
Hailed as a hero in Italy, Calipari died from a single shot to the head as he threw himself on top of Sgrena to save her life.
In newspaper interviews published on Friday, Sgrena said the American soldiers seemed upset when they realised what had happened.
"They looked stunned. Maybe one of them swore, then he called 'there's a dead one,'" Sgrena told her newspaper, the left-wing daily Il Manifesto. She told Milan daily, Corriere della Sera, that the soldiers walked over to the car and appeared "disheartened".
One Italian agent was killed by
the shooting on 4 March
Sgrena, who was wounded in the shoulder, said the soldiers pulled her out of the car and began cutting her clothes off. One tried to insert a drip in her wrist but failed, bruising it.
Sgrena has been distancing herself from comments she made shortly after her return to Rome on Saturday, in which she raised the possibility that the Americans targeted her because they disapproved of Italy's method of negotiating with captors.
Letter to Bush
President Carlo Azeglio Ciampi sent a letter to Bush on Wednesday pressing Bush on the need for "transparency and rapidity" in an US-Italian probe of the shooting while expressing appreciation for the American leader's "sincerity of the words of solidarity" over the tragedy.
Ciampi's letter, which his office released on Thursday, carried Wednesday's date and referred to a letter Bush had sent the Italian earlier in the week.
Giuliana Sgrena pleaded for her
life in a video
Ciampi told Bush he was taking note of "your assurances that the United States will launch an exhaustive joint probe between our two countries so that the facts of this tragedy are clarified in a thorough manner".
"The need for transparency and rapidity, which you yourself have expressed in an authoritative and sensitive way, is profoundly felt by the Italian people," wrote Ciampi.
The Italian Defence Minister Antonio Martino received assurances in a telephone call on Thursday evening from his US counterpart, Donald Rumsfeld, that the investigation would remove "any shade of doubt and pinpoint any responsibility".
Italy's top officials have ruled out an ambush, but the Italian version of what led to the shooting, based in part on testimony by the surviving agent, clashes on at least three points with the account furnished by US military authorities.
In Baghdad, the US Embassy said the troops who killed Calipari were part of extra security to protect US Ambassador John Negroponte. It was not known if Negroponte had passed through the checkpoint before the shooting. Premier Silvio Berlusconi has said that Calipari had informed the proper authorities that he was heading to the airport with the freed hostage.
However, La Repubblica and Corriere della Sera newspapers reported on Monday that US authorities knew Calipari and the other agent were in Iraq, but had not been told why they were there. Both newspapers cited a report given to Rome prosecutors by an Italian general in Iraq.
The US army's 3rd Infantry Division, which controls Baghdad, has said the vehicle was speeding and refused to stop despite hand and arm signals, flashing white lights and warning shots.