BAGHDAD, Iraq — U.S. troops opened fire last night on a car rushing an Italian journalist just freed by kidnappers to the Baghdad airport, wounding her and killing an Italian security agent who had helped negotiate her release. The incident could have political repercussions for one of the staunchest European supporters of the U.S. presence in Iraq.

Giuliana Sgrena, who had been held captive for a month, underwent surgery in a nearby U.S. military hospital to remove shrapnel from her shoulder and was expected to survive, Italian Prime Minister Silvio Berlusconi said in Rome.

Sgrena's car was traveling "at a high rate of speed" toward a checkpoint in darkness just before 9 p.m. and U.S. soldiers repeatedly "tried to warn the driver" to slow down before opening fire, the U.S. military said.

The statement 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 incident took place at Checkpoint 504-Camp Victory, near the Baghdad airport. The road from Baghdad to the airport is a dangerous throughway that has been the scene of numerous ambushes and car bombings. U.S. troops have frequently fired on cars approaching checkpoints around Iraq, for fear they might be suicide bombers.

A State Department official in Washington said the Italians did not tell either the U.S. Embassy in Baghdad or U.S. military commanders about Sgrena's release, even though a U.S. hostage coordinator had been working closely with them on the case.

Berlusconi, one of Washington's most loyal allies in the war in Iraq, said he had summoned U.S. Ambassador to Italy Mel Sembler for an explanation after hearing the news, which he said turned him "to stone."

"The behavior of the American soldiers, in such a serious incident, must be explained," Berlusconi said on national TV, as Italians who had begun to celebrate Sgrena's release were stunned at the turn of events. "Someone must take responsibility."

Berlusconi said secret-service agent Nicola Calipari, 50, died as he used his body to shield Sgrena. He said two other intelligence agents with Sgrena were also wounded.

The shooting came as a blow to Berlusconi, who has kept 3,000 troops in Iraq despite strong domestic opposition. The shooting was likely to set off new protests in Italy, where tens of thousands of people have regularly turned out on the streets to protest the Iraq war. Sgrena's newspaper was a loud opponent of the war.

"Nicola Calipari is the person we should thank for the freedom of Giuliana," Gabriele Polo, editor of Sgrena's left-wing newspaper, Il Manifesto, said in Rome. "Unfortunately, he was killed by an American patrol."

"Another victim of an absurd war," Alfonso Pecoraro Scanio, leader of the Green Party, told Italian news service 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 lives of civilians," said Piero Fassino, leader of the Democratic Party of the Left.

President Bush called Berlusconi and, in a five-minute conversation, expressed his regret, Bush spokesman Scott McClellan said.

"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 shooting might mean for participation by Italy or other countries in the coalition. "It's premature to get ahead of the investigation," he said.

The Italian agents from the state intelligence service apparently were dispatched to pick up Sgrena, 56, and had been involved in negotiations with her kidnappers, Italian officials said. It was not immediately revealed whether a ransom was paid to secure her release, as was thought to have been the case for at least some Italians previously taken hostage in Iraq.

Sgrena was shown in a videotape before her release, dressed in black and standing behind a table of fruit, apparently thanking her captors for letting her go. It was in marked contrast to a video released last month that showed her begging for her life in a crouched position, clasping her hands.

Sgrena, seized Feb. 4 after interviewing refugees from the Iraqi city of Fallujah, was one of nearly a dozen Italians who have been kidnapped in Iraq. At least two — a security guard and a journalist — were killed while others were released. Among those released were two female humanitarian-aid workers seized from their offices in Baghdad.

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.

Material from The Associated Press and The Washington Post is included in this report.