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.

Recruits are vetted, but records for criminal activity or past links to Saddam's regime are scattered and difficult to track down.

On Monday, U.S. troops arrested one current and two former ICDC soldiers for selling weapons to insurgents and carrying out bomb attacks on the homes of Iraqis cooperating with American forces in Tikrit, Saddam's hometown north of Baghdad.

Lt. Col. Steve Russell, a battalion commander in Tikrit, said infiltration is "not a major problem."

"In terms of the ICDC, this is where those that were not properly scrutinized may sometimes have their loyalties elsewhere. But these represent a very small number," he said.

Russell said information provided by ICDC officers led to the arrests, "proving that they don't want a negative reputation" for their forces.

As part of the recruiting process in Tikrit, Russell said he has local tribal leaders vouch for those wanting to join — and those who don't get the sheik's nod are turned down. The three ICDC members caught Monday were trained outside the area and would not have gone through that process, he said.

Senor called Tuesday's attack near Hillah, 35 miles south of Baghdad, a "targeted act of terrorism." Kimmitt, however, said it was not yet clear if the attackers knew their targets were coalition officials.

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AP correspondent Paul Garwood contributed to this report from Tikrit.