Two American soldiers were killed when a roadside bomb damaged their Humvee, the military said Friday. U.S. officials worried that Iraqi police -- not impostors in their uniforms -- may have been behind the shooting deaths of two coalition staffers and their translator.

The two soldiers died when the Humvee they were riding in struck a homemade bomb on Thursday northeast of Habbiniyah (searchin the Sunni Triangle (search), the heartland of the anti-U.S. insurgency.

A third soldier was wounded in the blast. The troops were part of the 1st Brigade Combat Team of Task Force All American (search), part of the 82nd Airborne Division.

The deaths brings to 556 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.

In Baghdad, a prominent supporter of radical Shiite cleric Muqtada al-Sadr was shot and killed in Baghdad's al-Shawafa district, Adnan al-Safi, a spokesman of the movement, said Friday.

The shooting of Kazim al-Sayed Musa al-Ghoriebi on Thursday came hours after a Sunni Muslim cleric was wounded in what he claimed was an assassination attempt against him that killed his son and son-in-law.

Nazem Khalaf, a cleric at the Rahman mosque in Abu Dsheer, a suburb in southwestern Baghdad, said that assailants drove up next to his car and opened fire.

In central Baghdad, thousands of Shiites rallied against the U.S. presence in Iraq, chanting "Kill America, kill America" and "Yes, yes to Islam."

Protesters hurled stones at a passing pair of armored civilian SUVs, of the type often used by Coalition workers or plain-clothes security officials, forcing the vechicles to back away. There were no injuries.

The American civilians slain Tuesday along with their translator were the first from the U.S. occupation authority to be killed in Iraq. The attack raised two possibilities: that guerrillas had adopted a new tactic of posing as police to carry out attacks, or that some members of the security forces being trained by U.S. troops are turning to violence.

One of the CPA workers was 33-year-old Fern Holland, a human rights expert from Oklahoma who worked on women's issues in the Hillah region where she was killed.

The other American killed was Robert J. Zangas, 44, of suburban Pittsburgh. Zangas went to Iraq last year with his Marine Corps Reserve unit and returned as a regional press officer with the coalition, according to his wife, Brenda Zangas.

The Americans and an Iraqi woman working as their translator were driving near Hillah, 35 miles south of Baghdad, when they were stopped at a checkpoint and killed by gunmen.

The attackers then took their car, their bodies still inside, according to the Polish military, which patrols the area. Polish troops stopped the car and arrested the five Iraqis inside.

Lt. Gen. Ricardo Sanchez, the U.S. commander in Iraq, said it was not yet known if the attackers were disguised as police or the real thing.

"We are very concerned about it," Sanchez said. "We know that this has gone on ... that there are some policemen that have done criminal acts in the past."

The U.S. military, which has been training Iraq's new police force, is trying "to ensure that they are truly serving their communities," he said.

Guerrillas have not been widely known to use police disguises -- and the attack on the coalition employees near Hillah could signal a new tactic. Roads across Iraq are dotted with checkpoints manned by Iraqi police or coalition troops, particularly at the entrances to towns.

There has been at least one other case of members of the Iraqi security forces working with insurgents. On Monday, U.S. forces captured two members of the Iraqi Civil Defense Corps who were "suspected of conducting anti-coalition activities," the military said.

The defense corps was set up by the U.S. military as an internal security force. It and the Iraqi police force are supposed to gradually take greater responsibility in battling the insurgency after the coalition hands over sovereignty to a new Iraqi government on June 30 -- though Sanchez cautioned that the handover of security powers would take time.

Early Friday, soldiers near Uja, arrested three Iraqis, including two wanted for weapons trafficking, after they raided a farm near the town, said Lt. Col. Jeff Sinclair, commander of the 1st Battalion, 18th Infantry, 1st Infantry Division.