This month, the Pentagon and the Iraqi government are finally closing the books on the program that handled funding for reconstruction in postwar Iraq. But despite years of investigations, US defence officials still cannot say what happened to $US6.6 billion ($6.3 billion) of the cash. Federal auditors are now suggesting that some or all of the cash may have been stolen, not just mislaid in an accounting error. After the US-led invasion in March 2003, the Bush administration flooded Iraq with so much cash that a new unit of measurement was born. Pentagon officials determined that one giant C-130 Hercules cargo plane could carry $US2.4 billion in shrink-wrapped bricks of $US100 bills. They sent an initial full planeload of cash followed by 20 other flights by May 2004 in a $US12 billion haul that US officials believe to be the biggest ever international cash airlift. Stuart Bowen, special inspector-general for Iraq reconstruction, said the missing $US6.6 billion might be ''the largest theft of funds in national history''. Iraqi officials are threatening to go to court to reclaim the money, which came from Iraqi oil sales, seized Iraqi assets and surplus funds from the United Nations' oil-for-food program. Pentagon officials have contended for the past six years that they could account for the money if given enough time to track down the records. But repeated attempts to find the documentation, or better yet the cash, were fruitless.