The assault on the night of Jan. 20 stunned U.S. officials with its planning and sophistication. A column of SUVs filled with gunmen who posed as an American security team passed through Iraqi police checkpoints at a provincial headquarters in the Shiite holy city.

Within a few minutes, the attackers killed one American, wounded three and abducted four. The captives were later found shot to death; the gunmen escaped.

"(The American) defense hinged on a level of trust that … early warning and defense would be provided by the Karbala Iraqi police. This trust was violated," the report dated Feb. 27 says.

The information is contained in an investigative file made available to USA TODAY and authenticated by the Army.

The attack has drawn special scrutiny from Pentagon officials because of the unprecedented breach of security and the insurgents' tactics.

The investigation reveals several new details about the assault, including:

•Iraqi police suddenly vanished from the government compound before the shooting started.

•Attackers, evidently briefed on how U.S. forces would defend themselves, bottled up more than three dozen soldiers in a barracks and headquarters complex using a combination of smoke and fragment grenades and satchel charges to blow up Humvees.

•Gunmen knew exactly where to find and abduct U.S. officers.

•Iraqi vendors operating a PX and barbershop went home early.

•A back gate was left unlocked and unguarded.

Investigators recommended several changes to toughen defensive positions, including the installation of closed-circuit cameras to provide better early warnings, "duress devices" that can allow overrun outposts to signal headquarters, and requirements that any arriving convoy provide identification.

This month, Army officials publicly alleged that Iran played a direct role in the Karbala attack.

The Quds Force, an elite unit of Iran's Islamic Revolutionary Guards, helped plan and direct it with Iraqi militants, Brig. Gen. Kevin Bergner, a military spokesman, said at a news conference.

The Quds Force, he said, supplied Shiite militias with weapons and up to $3 million a month in aid.

Hezbollah, a Shiite militant group, is also working in Iraq and training militia groups, Bergner said.

The Iranian involvement in the Karbala attack may have even included planning with the Iraqi police who had colluded with the attackers.

Several U.S. troops who survived the attack later told investigators that they believe some gunmen were allowed to blend in among Iraqi police inside the headquarters compound hours before the assault, according to interviews included in the report.

"It appears an inside assault force was pre-staged," the report says.

American soldiers also told investigators that, as the assault ended, they saw an Iraqi police commander in the complex talking on his cellphone and laughing.

The infiltration of local police units by sectarian militias "remains a significant problem," according to a Pentagon status report on Iraq issued in June.

Such collusion is almost unavoidable, experts say.

"There's no way you can fight this kind of war without significant problems with infiltrators. It was a major problem in Vietnam. It was a major problem in Korea. It's a problem in any kind of campaign where you are working closely with local forces," says Anthony Cordesman, a military analyst and Iraq expert withthe Center for Strategic and International Studiesin Washington.

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