Updated

The Italian intelligence agent killed by American forces in Iraq had U.S. military authorization for his operation to win the release of a hostage, Premier Silvio Berlusconi (search) said Wednesday.

In his first major address since Friday's shooting strained relations between Washington (search) and one of its biggest allies, Berlusconi told Italy's Senate that the car carrying agent Nicola Calipari and newly freed hostage Giuliana Sgrena (search) stopped immediately when a light was flashed.

The U.S. military has said the Americans used hand and arm signals, flashed white lights and fired warning shots to try to get the car to stop.

The idea that Calipari was killed by friendly fire is "painful" to accept, Berlusconi said. But he reassured lawmakers: "The United States has no intention of evading the truth."

"I'm sure that in a very short time every aspect of this will be clarified," he said.

Berlusconi, a staunch supporter of U.S. President George W. Bush (search) and the U.S.-led campaign in Iraq, is struggling to balance his decision to keep 3,000 troops in the country against heavy anti-war sentiment among ordinary Italians.

His 10-minute address made no mention of ransom payments to win Sgrena's release. Some Italian officials have suggested a ransom was paid, but there has been no official confirmation.

Bush sent a letter to his Italian counterpart renewing his promise for a swift and thorough investigation into the killing, the Italian president's office said Wednesday.

Italy has called the shooting an "accident," echoing the White House's characterization of the death, but has disputed the U.S. version of events and demanded that Washington shed light on the incident and punish those responsible.

"The case of friendly fire is certainly the most painful to bear. It feels like an injustice beyond any sentiment. It's something unreasonable," Berlusconi said.

"When Italian citizens have been victims of kidnappings, the government has always acted by following two directives: It has always rejected political blackmail, while at the same time activating all the political, diplomatic and intelligence channels to obtain the release of our nationals," he said.

Calipari was shot and killed Friday as he headed to Baghdad's airport after securing the release of Sgrena, who had been kidnapped on Feb. 4. Sgrena and another intelligence officer in the vehicle were wounded.

In his letter to Italian President Carlo Azeglio Ciampi, Bush called the shooting a "terrible tragedy" and expressed his solidarity, Ciampi's office said in a statement.

"In his letter, President Bush assures President Ciampi that the United States will move toward a swift and exhaustive joint investigation" between Rome and Washington, the statement added.

The U.S.-led coalition in Iraq announced Tuesday it was ordering an investigation into the shooting, to be led by a U.S. brigadier general with Italian officials' participation.

On Friday, shortly after the attack, Bush called Berlusconi and promised a full investigation.

Berlusconi's address to the Senate came a day after Foreign Minister Gianfranco Fini went before lawmakers to dismiss the idea that the shooting was the result of an ambush, but also to dispute that the vehicle was speeding up and was ordered to stop before the shooting, as Americans have indicated.

The possibility of an ambush had been raised by Sgrena, who in some of her first comments since being released contended that the United States disapproved of Italy's method of negotiations with kidnappers.

Sgrena, who works for the communist daily Il Manifesto, tried to distance herself from the ambush hypothesis Tuesday.

"I never said that they wanted to kill me, but that the mechanics of what happened are that of an ambush," she told state TV.