Italian journalist Giuliana Sgrena arrived back in Rome on Saturday, the day after being freed and subsequently sustaining injuries when her car was fired upon by American troops. Among those on hand to greet her at the airport was Italian Prime Minister Silvio Berlusconi.

On Friday, U.S. forces fired on a car carrying the former hostage, who had been freed after spending a month in captivity. She was injured in the incident and an Italian intelligence agent was killed, according to military sources.

The shooting reportedly occurred at a coalition checkpoint in western Baghdad. The car was traveling at high speeds, prompting American troops warn the driver to stop by using "hand and arm signals, flashing white lights, and firing warning shots," the military said. When that didn't work, GIs shot into the engine block, according to the military.

President Bush expressed regret over the deadly attack, the White House said Friday afternoon.

The editor of freed hostage Giuliana Sgrena's (search) newspaper Il Manifesto, Gabriele Polo, said the intelligence agent was killed when he threw himself over Sgrena to shield her from U.S. fire, according to the Apcom news agency in Italy. The Italian intelligence officer had helped negotiate Sgrena's release.

Elsewhere on 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."

Prime Minister Silvio Berlusconi (search), an ally of the United States who has kept troops in Iraq despite public opposition at home, demanded an explanation about the roadblock shooting from the U.S. ambassador, Mel Sembler.

"Given that the fire came from an American source I called in the American ambassador," Berlusconi told reporters before the U.S. statement acknowledging that coalition forces shot at the vehicle. "I believe we must have an explanation for such a serious incident, for which someone must take the responsibility."

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."

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.

The U.S. military said "at approximately 8:55 p.m. tonight, coalition forces assigned to the multinational force Iraq fired on a vehicle that was approaching a coalition checkpoint in Baghdad at a high rate of speed."

Berlusconi identified the dead intelligence officer as Nicola Calipari (search) and said he had been at the forefront of negotiations with the kidnappers. The premier said Calipari had been involved in the release of Italian hostages in the past.

U.S. troops took Sgrena to an American military hospital, where shrapnel was removed from her left shoulder, he said.

Sgrena, 56, who worked for the leftist Il Manifesto, 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.

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 (search), 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.

The circumstances of Sgrena's release were unclear.

The Italian government announced earlier Friday that Sgrena had been freed, prompting expressions of joy and relief from officials and her family.

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.

FOX News' Bret Baier and Nick Simeone and The Associated Press contributed to this report.