Iraq's police force is recruiting insurgents and former criminals to its ranks, according to a report released by the US defence department.
There are serious questions about the effectiveness of the forces
It blames poor vetting procedures and recommends that the quality of records at Iraq's interior ministry be checked.
US-run training programmes, in which more than 60,000 Iraqi recruits have taken part, are only a qualified success, the Pentagon report says.
An earlier report found only 50% of battalions able to combat insurgents.
The formation of an effective police force is a key element of attempts to combat the insurgency in Iraq, in which hundreds of police officers and would-be recruits have been killed.
The report praises the work of the police during January's election, and says police officers are increasingly visible on the streets.
But it says many new recruits are illiterate, have criminal records or are physically disabled.
"Inducting criminals into the [Iraqi police] is a continual concern," the report says, quoted by AP news agency.
"Even more troubling is infiltration by intending terrorists or insurgents. There is sufficient evidence to conclude that such persons indeed are among the ranks of the [police]."
The report adds that coalition personnel are ineffective as recruiters, and focus on quantity rather than quality.
"There is a perception that training programmes have produced 'cannon fodder' - numbers of nominal policemen incapable of defending themselves, let alone the Iraqi public," it says.
Current plans are for a 135,000-strong force.
US-led forces are expected to stay in the country until Iraqi security forces are able to withstand the insurgent on their own.