FBI agents on Thursday investigated the slayings of two American coalition staffers and their translator by gunmen dressed as police. An American soldier was killed by a roadside bomb north of the capital.

In the southern city of Basra, gunmen killed two Iraqi women working in a laundry for the U.S.-led coalition, officials said Thursday. The women, who were sisters, were riding home in a taxi late Wednesday when gunmen stopped the vehicle and shot them, a coalition official said.

The women worked in a laundry for the U.S. company Kellogg Brown & Root (search), which has a contract to provide logistical services for the coalition and military, a British Ministry of Defense official said.

The motive for the attack was not immediately known. Guerrillas have targeted Iraqis working with the U.S.-led occupation. Also, Basra (search), which is patrolled by the British military, has seen a number of killings blamed on Shiite militias enforcing Islamic law.

The military said Thursday detained 30 people during separate raids targeting Iraqis wanted for attacking coalition forces in the north. The raids were around Kirkuk (search) and Baqouba (search), Army spokesman Maj. Neal O'Brien said.

Among those arrested was Zachair George, a man wanted by American forces for attacking coalition troops with homemade bombs, O'Brien said. Another wanted bombmaker was detained in Muqdadiyah, northeast of Baqouba.

Soldiers also confiscated rocket-propelled grenades, bombmaking equipment, guns and ammunition during the raids.

The two Americans and their translator killed Tuesday night were the first civilian staffers from the U.S.-led occupation authority to be killed in Iraq. Their deaths raised the worrisome prospect that guerrillas have taken a new tactic of posing as police.

"We are very concerned about it," Lt. Gen. Ricardo Sanchez, the top U.S. commander in Iraq, said in Baghdad.

Sanchez said he did not believe the gunmen were real policemen, although "we know that this has gone on ... that there are some policemen that have done criminal acts in the past."

An FBI team began its investigation in the Hillah area, 35 miles south of Baghdad, where the killings occurred, Sanchez said.

The gunmen stopped the vehicle at a checkpoint outside Hillah and shot the three passengers, Polish Col. Robert Strzelecki said from the Camp Babylon headquarters of the Polish-led multinational force in Iraq.

Polish troops patrolling the area arrested five Iraqis who were in the Americans' vehicle with the three bodies still inside. The identities of the victims have not been released pending notification of their families. The Americans were Defense Department employees.

It was not known whether the gunmen were specifically targeting coalition officials, a coalition spokesman said. Dan Senor also said some reported details of the attack were incorrect, but he would not elaborate.

It also was unclear if the Americans were traveling with security, and coalition guidelines discourage staffers from movements after dark. The roads around Hillah have seen a number of attacks on vehicles, some fatal, including the Feb. 14 killing of an American civilian.

The military said Thursday that an American soldier was killed and two others wounded when a homemade bomb exploded in Baqouba, north of Baghdad, a center of anti-U.S. insurgent activity.

None of the soldiers were identified. All were part of the 652nd Engineering Battalion and based in Baqouba.

The latest death brings to 554 the number of U.S. service members who have died since the United States launched the Iraq war in March. Most have died since President Bush declared an end to active combat May 1.

Also Thursday, a roadside bomb detonated as a U.S. military convoy of nearly 50 vehicles passed by on a main street in Fallujah, killing one Iraqi and wounding two others, police Lt. Omar Ismail said.