In his first major address since Friday's shooting strained relations between Washington and one of its biggest allies, Berlusconi told Italy's Senate that the car carrying agent Nicola Calipari and former hostage Giuliana Sgrena stopped immediately when a light was flashed.
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.
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.
U.S. President George W. 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, a staunch supporter of war in Iraq, has sent 3,000 troops to the southern part of the country after the ouster of Saddam
Whatever official conclusions are reached, the controversy over Nicola Calipari's death is testing the limits of the Italian government's unwavering support for the U.S., reports CBS News Correspondent Allen Pizzey.
Nearly 80 percent of Italians were already opposed to the Iraq operation, and many people said they felt anger towards the U.S. during Calipari's state funeral on Monday, reports Pizzey.
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.