U.S. forces fired on the vehicle Calipari and Sgrena were riding in as it approached a coalition checkpoint on airport road in Baghdad.
The follow-on investigation to the Multinational Division Baghdad commander's initial inquiry into the incident is expected to take about three to four weeks to complete. The command is working closely with the U.S. Embassy, and Italian officials have been invited to participate.
Multinational Force Iraq issued a statement extending the commands deepest sympathies to Caliparis family.
The command also issued a statement saying it regretted the March 4 death of Jr. Sgt. Gardi Gardev from Bulgaria. The Bulgarian government said March 7 the death was the result of U.S. friendly fire.
Officials from 1st Corps Support Command are investigating the incident. More details will be provided when they are available.
MNFI said it is committed to working with its Bulgarian counterparts to determine the cause of the soldiers death.
According to  Lieutenant Colonel Sevastian Dobrev, a Bulgarian defense ministry official, U.S. forces killed a Bulgarian soldier in what the army calls a 'friendly fire' incident in Iraq on March 4. The official, who has direct knowledge of the investigation, told ''Voice of America News'' on Wednesday that Bulgarian military police in Iraq are convinced that 30-year-old Junior Sergeant Gardi Gardev was killed when several American troops opened fire on his convoy.

Dobrev said the shooting took place at night in a desert area, when Bulgarian troops and U.S. forces at a nearby radio post were not able to recognize each other. He said the trouble began after Bulgarian troops stepped out of their armoured vehicles to fix a problem with their communication equipment, about 160 kilometers southeast of Baghdad.

"At that moment one Iraqi car tried to pass them," he explained. "They tried to stop it, but the car did not want to stop. They shot two times in the north direction in that area. A few seconds after that our coalition partners from the United States from this radio post opened fire."

Lieutenant-Colonel Dobrev said two bullets hit machine gunner Gardi Gardev during the incident. He said colleagues tried to revive Sergeant Gardev, while an American helicopter was called in to take him to a hospital.

However, the helicopter did not land and another emergency vehicle arrived too late to save his life. Mr. Dobrev said the inexperience of American forces at the recently built radio post in the area contributed to what the army calls a "friendly fire incident."

"Our guys know exactly where they are deployed on the ground. They signed this place on their maps," he said. "But there was no contact, any kind of contact, between the Bulgarian battalion and the American radio post there."

Mr. Dobrev said Bulgarian and American military officials will meet in Baghdad Saturday to discuss how to improve coordination and the outcome of investigations by Bulgarians and two American teams.

He denied the incident has increased anti-American sentiment in Bulgaria, which is among several Eastern European countries supporting the US-led efforts in Iraq. Bulgaria has a 460-strong battalion in the Iraqi city of Diwaniya, serving under Polish command, and its mandate expires mid-2005.

Bulgarian soldier Gardev, who was buried Tuesday in his home village of Dolno Sahrane, was posthumously awarded the rank of candidate officer and received a medal for his apparent bravery in Iraq.

He died around the same time that American soldiers shot and killed an Italian intelligence agent, while also wounding an Italian journalist the agent had helped to rescue from insurgents. U.S. officials have expressed their regret over both incidents. On March 7, the Bulgarian government stated that Jr. Sgt. Gardi Gardev was killed by U.S. troops. 

Officials from 1st Corps Support Command of the Multi-national Force Iraq are investigating the incident, and have said that they are committed to working with Bulgarian officials to discover what happened.