In the latest assault on Iraq's U.S.-trained security forces, gunmen killed four people in two separate attacks on police south of Baghdad on Saturday.

A senior U.S. official, meanwhile, said investigators were studying videotape of Iraqis mutilating the bodies of four American contract workers killed Wednesday in Fallujah (search), trying to identify participants.

The charred remains of the Americans were dragged through the streets for hours after insurgents ambushed their vehicles. Two corpses were hung from a bridge.

There was no sign of any U.S. military activity in the Fallujah area to suggest retaliatory action was imminent. U.S. administrator L. Paul Bremer (search) has said those who killed the four civilians and burned their bodies "will not go unpunished."

In the first attack on police Saturday, the department chief of Mahmoudiya, 20 miles south of Baghdad, was driving from the capital to his home when gunmen killed him and his driver, police Lt. Ala'a Hussein said.

Not long afterward, six attackers shot at a four-man police patrol in Mahmoudiya, killing one and wounding three, police officer Khaldoon al-Gurairi said. A 60-year-old bystander was also killed.

Guerrillas often target police because they view them as collaborators with the U.S.-led occupation. Also they make easier targets because they are less well-armed and protected than the U.S. troops.

More than 350 policemen have been killed by shootings and suicide bombings since the ouster of Saddam Hussein's regime last year, and some Iraqi officials put the toll much higher. On March 24, nine police recruits were killed when gunmen shot up their vehicle in southern Babil province.

The violence has not stopped Iraqis from seeking police jobs, however. In the southern city of Basra, unemployed men demanding jobs on the force clashed with Iraqi security forces Saturday, police Col. Ali Kahdum said. He said protesters raided and looted the city's central police office. Three protesters were hurt.

There have been several protests in recent weeks in Basra by men demanding jobs as policemen. With tens of thousands of unemployed in the region and elsewhere in Iraq, a policeman's minimum monthly wage of $120 is a high incentive to sign up. The salary is almost twice that of newly recruited teachers.

Also Saturday, an explosion near a U.S. military convoy near Khalis, 40 miles northeast of Baghdad, wounded a civilian and damaged a Humvee, Iraqi officials said. It was not clear if Americans were hurt.

In western Baghdad on Saturday, a rocket slammed into a house in a residential suburb, killing two people and wounding four, said Jamil Ibrahim, a doctor at Yarmouk hospital.

About 5,000 members of radical Shiite Muslim cleric Muqtada al-Sadr's (search) self-styled militia, the al-Mahdi Army, paraded in Sadr City, a mainly Shiite district in eastern Baghdad, on Saturday. The unarmed black-clad militiamen carried portraits of the cleric. They marched past a reviewing stand where Muslim clerics acknowledged their salutes.

Hundreds of Iraqis lined the route. Sharpshooters from the militia were stationed on rooftops. Al-Sadr has been an outspoken critic of the U.S.-led occupation, but has not called for attacks on the occupying forces.

A senior aide to al-Sadr, Mustafa al-Yaqoubi, was arrested in a raid on his house in Najaf before dawn Saturday, said Sheik Haider al-Mousawi, another al-Sadr followers. U.S. officials could not confirm the arrest and Spanish forces based in the city denied detaining him.

Al-Sadr's weekly newspaper was shut by U.S. officials on Thursday, provoking an enormous anti-American outpouring.

Spanish forces arrested Mustafa al-Yaqoubi (search), a senior aide to al-Sadr, in a raid on his house in the southern city of Najaf before dawn Saturday, said Sheik Haider al-Mousawi, an al-Sadr backer. It was not known why he was arrested.