BAGHDAD, Iraq — Four Iraqis suspected of killing a pair of American officials and their translator appear to be active police officers working with a Saddam Hussein loyalist, a top U.S. military official said Friday, raising concerns that insurgents are infiltrating Iraqi security forces being trained by U.S. forces.

The four were caught along with a former officer from the Saddam-era police forces and a civilian after the slayings Tuesday of the two U.S.-led coalition staffers and an Iraqi woman south of Baghdad, Maj. Gen. Mark Kimmitt said.

U.S. troops have been setting up Iraqi police and other security forces, intending to gradually put them on the frontlines against guerrillas.

Coalition spokesman Dan Senor called the policemen's role in the attack "an exception" and defended what he called a "robust" process of vetting police recruits to try to uncover criminal pasts or links to Saddam's regime. "But it is not perfect," he said. "Individuals slip through the cracks. We act to identify it and remove them immediately."

FBI experts were investigating the attack that killed the three, amid conflicting reports over the shooting outside the town of Hillah. Polish troops patrolling the region said the police stopped the victims' car at a checkpoint and shot them to death.

Kimmitt, however, said the attackers may have been in a second car that ran the coalition staffers off the road.

Meanwhile, the military announced Friday that two U.S. soldiers were killed and a third wounded when their Humvee struck a roadside bomb northeast of Habbiniyah in the Sunni Triangle, heartland of the anti-U.S. insurgency.

The latest deaths bring 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.

The American civilians slain Tuesday with their translator were the first from the U.S. occupation authority to be killed in Iraq. One was Fern Holland, 33, a human rights expert from Oklahoma who worked on women's issues in the Hillah region. The other was a regional press officer, Robert J. Zangas, 44, of suburban Pittsburgh.

Kimmitt said four of the six men in custody, caught together in the same car soon after the attack, had current police identification. Investigators were examining whether they were authentic but "we believe they are valid," he said.

U.S. officials have trained more than 70,000 Iraqi police officers, as well as some 25,000 members of the Iraqi Civil Defense Corps, or ICDC, in a matter of months. New recruits undergo an eight-week training program, while veteran officers have three weeks of training on new techniques and democratic principles, Senor said.