WASHINGTON, July 12 (Reuters) - A U.S. Army investigation has concluded that Iraqi police assisted insurgents in an assault in the Shi'ite holy city of Kerbala in January that killed five U.S. soldiers, USA Today reported on Thursday.

USA Today said the information was contained in an investigative file made available to the newspaper and authenticated by the Army.

During the attack, guerrillas posing as Americans entered a government compound in Kerbala, killed a U.S. soldier and drove away with four others whom they shot and killed later.

"(The American) defense hinged on a level of trust that ... early warning and defense would be provided by the Kerbala Iraq police. This trust was violated," said the army report, dated Feb. 27.

The paper said the attack had attracted special scrutiny by Pentagon officials because of the unprecedented breach of security and the tactics used by the insurgents.

Among details included in the investigation were that Iraqi police vanished from the government compound before the attack, that gunmen knew exactly where to find and abduct U.S. officers and that a back gate was left unlocked and unguarded.

Some U.S. soldiers who survived the assault told investigators they believed some attackers were allowed to blend in among Iraqi police inside the compound in the hours before the attack, the report said.

It added that soldiers also told investigators they saw an Iraqi police commander in the compound talking on a cell phone and laughing as the assault ended. (Writing by Peter Cooney, editing by Catherine Evans; World Desk Americas, 1-202-898-8457))

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