George W Bush has pledged to fully investigate the shooting of an Italian hostage who came under fire after being rescued from her kidnappers in Iraq.
Giuliana Sgrena was interviewing people when she was abducted
An Italian security agent who had helped rescue journalist Giuliana Sgrena was killed in the shooting.
Italy's prime minister Silvio Berlusconi has demanded to know why US troops at a checkpoint fired on the car carrying Ms Sgrena to safety.
Ms Sgrena has been discharged from hospital and is flying back to Italy.
Mr Berlusconi, one of President George W Bush's staunchest allies, held an hour-long meeting with the US ambassador in Rome and afterwards said he wanted to know who was responsible.
"The prime minister expects that, in the spirit of the particular friendship that characterises relations between Italy and the United States, the US government leaves no stone unturned to shed light on what happened," he said in a statement.
Mr Bush spoke to Mr Berlusconi during a five-minute telephone call during which he expressed regret about the incident.
He "assured Prime Minister Berlusconi that it would be fully investigated," according to White House spokesman Scott McClellan, who added that the US were co-operating closely with Italian authorities.
An Italian military plane has been sent to Baghdad to collect Ms Sgrena.
On Friday evening, the car carrying Giuliana Sgrena and three Italian security agents towards the airport came under fire as it approached, at speed, a checkpoint in western Baghdad.
US military officials in Baghdad say they tried to stop the car - making hand signals, flashing lights and firing warning shots - before firing at the vehicle's engine.
Anger and anguish
Nicola Calipari, an agent who had helped negotiate Ms Sgrena's release, was hit as he tried to shield her body from the gunfire.
Ms Sgrena was injured in the shoulder during the shooting and was later treated by coalition forces.
Ms Sgrena's colleagues cheered the initial news of her release
In the Italian capital, the ancient Roman colosseum was lit up to celebrate the homecoming of the award-winning journalist, kidnapped over a month ago.
But the celebratory mood on the streets and in the offices of her left-wing newspaper, Il Manifesto, was tempered by anger and anguish as news of what had happened in Baghdad emerged.
"It's incredible that a man who was busying himself with the difficult task of saving a life was killed by those who say they are in Iraq to safeguard the life of civilians," said Piero Fassino, leader of the Democratic Party of the Left.
Communist Senator Gianfranco Pagliarulo urged people to turn up outside the US consulate in Milan to demonstrate against the attack.
Earlier, a video was broadcast on the Arabic al-Jazeera satellite network showing an apparently healthy Ms Sgrena, 56, thanking her captors for the way they treated her during her month in captivity.
A little-known militant group, Islamic Jihad Organisation, had said it kidnapped her and demanded that Italy withdraw its troops from Iraq.
The same group said in September it had killed two Italian aid workers, Simona Torretta and Simona Pari - but they were later released by another organisation.