It was the first exchange of fire involving U.S. troops in Baghdad since the Aug. 31 deadline for formally ending the combat mission, and it showed that American troops remaining in the country are still being drawn into the fighting.

The attack also made plain the kind of lapses in security that have left Iraqis wary of the U.S. drawdown and distrustful of the ability of Iraqi forces now taking up ultimate responsibility for protecting the country.