The Americans were the first U.S. civilians from the occupation authority to be killed in Iraq.

Farther south, Iraqi police clashed with a Shiite Muslim militia during a raid on a building in a gunbattle that killed four policemen and wounded two.

L. Paul Bremer, the top administrator in Iraq, has requested that the FBI investigate the slayings of the Americans late Tuesday on a road outside the town of Hillah, 35 miles south of Baghdad, said Dan Senor, spokesman for the U.S.-led coalition.

It was not known whether the gunmen were specifically targeting coalition officials. "We're starting to form views on that," Senor said.

And it was unclear if the Americans were traveling with security. Coalition guidelines discourage the movement of staffers after dark. The roads around Hillah have seen a number of attacks on vehicles, some of them fatal.

An officer with the Polish military, which patrols south-central Iraq, said the gunmen were disguised as policemen and stopped the Americans' car at a checkpoint. The attackers shot the passengers and took the vehicle, Col. Robert Strzelecki said.

Polish troops later intercepted the car, arrested five Iraqis in it and found the bodies inside, said Strzelecki, speaking from the Camp Babylon headquarters of the Polish-led multinational force in Iraq.

Senor said some reported details of the attack were incorrect, but would not elaborate. He did not identify the dead, pending notification of relatives.

The Americans, who were Defense Department employees, were the first U.S. civilians from the Coalition Provisional Authority to be killed in Iraq, Senor said.

An Army colonel working for the coalition was killed Oct. 26, when insurgents fired a barrage of rockets at Baghdad's Al-Rasheed hotel while Deputy Defense Secretary Paul Wolfowitz was visiting. Fifteen people were wounded, and Wolfowitz escaped unharmed.

Civilian contractors have also been killed. Since the war began, 553 U.S. service members have died in Iraq, 379 of them from hostile action. Since May 1, when President Bush declared major combat operations in Iraq over, 264 U.S. troops have been killed by the insurgency thought to be led by Saddam Hussein loyalists or foreign fighters.

In the southern city of Nasiriyah, Iraqi police tried Tuesday night to raid a building where a Shiite militia was holding two civilians, a coalition spokesman said. In a gunbattle, four Iraqi policemen were killed and two wounded.

The standoff ended when Italian security forces stormed the building, rescued the civilians and arrested eight militia members, the spokesman said. One Italian Carabinieri officer was slightly injured.

The Shiite militia, known as Citizens' Security Group, acts as a security force for some Shiite political parties. Such militias, which in some towns try to enforce a version of Islamic law, often have tense relations with the U.S.-trained Iraqi police force.

Also Wednesday:

- Gunmen killed two police officers and critically wounded a third at a restaurant in the northern town of Qaim, near the Syrian border, police said.

- A fire broke out in an oil pipeline south of Baghdad, and it appeared to be sabotage, said firefighter Saleh Jabbar. The extent of the damage was not immediately known on the line, leading from southern fields to the Al-Doura processing plant on the edge of Baghdad.

- The Voice of America reported that an Iraqi translator for the VOA was shot to death along with his mother and daughter on March 5 by unknown assailants. Selwan Abdelghani Medhi al-Niemi was driving home from a relative's house when he was killed, the VOA said. No motive was known.

- North of Baghdad, a bomb went off in the town of Baqouba near the offices of Iraq's largest Shiite party, wounding two people, said Haithem al-Husseini, a spokesman for the Supreme Council for the Islamic Revolution in Iraq.