BAGHDAD, Iraq -- American troops fired on a car rushing Italian journalist Giuliana Sgrena to freedom on Friday after a month in captivity, killing the Italian intelligence officer who helped negotiate her release and wounding the reporter in another friendly-fire tragedy at a U.S. checkpoint.
Prime Minister Silvio Berlusconi, an ally of the United States who has kept Italian troops in Iraq despite public opposition at home, demanded an explanation "for such a serious incident, for which someone must take the responsibility."
President Bush expressed regret and promised to investigate, the White House said.
The U.S. military said the car was speeding as it approached a coalition checkpoint in western Baghdad at 8:55 p.m. It said soldiers shot into the engine block only after trying to warn the driver to stop by "hand and arm signals, flashing white lights, and firing warning shots."
The Americans said two people were wounded, but Berlusconi said there were three -- Sgrena and two intelligence officers. One of the officers was in serious condition with an apparent lung injury, according to the Apcom news agency in Italy. The U.S. military said Army medics treated a wounded man but that "he refused medical evacuation for further assistance."
The intelligence agent was killed when he threw himself over Sgrena to protect her from U.S. fire, Apcom quoted Gabriele Polo, the editor of the leftist Italian newspaper Il Manifesto, as saying. Sgrena works for Il Manifesto.
Berlusconi identified the dead intelligence officer as Nicola Calipari and said he had been at the forefront of negotiations with the kidnappers. The prime minister said Calipari had been involved in the release of other Italian hostages in Iraq in the past.
U.S. troops took Sgrena to an American military hospital, where shrapnel was removed from her left shoulder. Apcom said Sgrena was fit to travel and would return to Rome today.
Sgrena, 56, was abducted Feb. 4 by gunmen who blocked her car outside Baghdad University. Last month, she was shown in a video pleading for her life and demanding that all foreign troops -- including Italian forces -- leave Iraq.
Berlusconi said he had been celebrating Sgrena's release with the editor of Il Manifesto, and with Sgrena's boyfriend, Pier Scolari, when he took a phone call from an agent who informed them of the shooting.
"It's a shame that the joy we all felt was turned into tragedy," Berlusconi said.
The shooting came as a blow to Berlusconi, who has kept 3,000 troops in Iraq despite strong opposition in Italy.
The shooting was likely to set off new protests in Italy, where tens of thousands have regularly turned out on the streets to protest the Iraq war. Sgrena's newspaper was a loud opponent of the war.
"Another victim of an absurd war," Alfonso Pecoraro Scanio, leader of the Green Party, told Apcom.
"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.
Bush called Berlusconi and, in a five-minute conversation, expressed his regret about the incident, Bush spokesman Scott McClellan told reporters aboard Air Force One on Friday night.
"The president assured Prime Minister Berlusconi it would be fully investigated," McClellan said. "I think he appreciated that."
McClellan wouldn't comment on what the incident might mean for participation by Italy or other countries in the coalition.
"It's premature to get ahead of the investigation," McClellan said.
Iraqis have reported numerous incidents where confusion at U.S. checkpoints has led to U.S. soldiers killing innocent civilians.
In a 2003 friendly-fire incident involving Italians, American soldiers in northern Iraq shot at a car carrying the Italian official heading up U.S. efforts to recover Iraq's looted antiquities. Pietro Cordone, the top Italian diplomat in Iraq, was unhurt, but his Iraqi translator was killed.
Cordone, also the senior adviser for cultural affairs of the U.S. provisional authority, was traveling on the road between Mosul and Tikrit when his car was fired on at a U.S. roadblock, according to an Italian Foreign Ministry official.
Also Friday, four U.S. soldiers were killed west of the capital in sprawling Anbar province, where American troops launched a massive sweep two weeks ago to root out insurgents, the military said. The soldiers, assigned to the 1st Marine Expeditionary Force, were killed "while conducting security and stability operations."
The circumstances of Sgrena's release from captivity were unclear.
The Italian government announced earlier Friday that Sgrena had been freed, prompting expressions of joy and relief from officials and her family.
Foreign Minister Gianfranco Fini expressed "great joy and enormous satisfaction," the ANSA news agency said.
The reporter's father was so overwhelmed by the news that he needed assistance from a doctor, ANSA said. "This is an exceptional day," Franco Sgrena was quoted as saying.
At Il Manifesto's offices, reporters toasted the release with champagne.
On Feb. 19, tens of thousands of demonstrators marched through Rome waving rainbow peace flags to press for Sgrena's release. Il Manifesto and Sgrena's boyfriend organized the march.
About 200 foreigners have been abducted in Iraq in the past year, and more than 30 of the hostages were killed.
Another European reporter, Florence Aubenas, a veteran war correspondent for France's leftist daily Liberation, is still being held in Iraq. Aubenas and her interpreter, Hussein Hanoun al-Saadi, disappeared nearly two months ago.
Associated Press writer Angela Doland contributed to this report from Rome.