WASHINGTON — After the U.S.-led invasion of Iraq in March 2003, the Bush administration flooded the conquered country with so much cash to pay for reconstruction and other projects in the first year that a new unit of measurement was born.

Pentagon officials determined that one giant C-130 Hercules cargo plane could carry $2.4 billion in shrink-wrapped bricks of $100 bills. They sent an initial full planeload of cash, followed by 20 other flights to Iraq by May 2004 in a $12 billion haul that U.S. officials believe to be the biggest international cash airlift of all time.

This month, the Pentagon and the Iraqi government are finally closing the books on the program that handled all those Benjamins. But despite years of audits and investigations, U.S. defense officials still cannot say what happened to $6.6 billion in cash – enough to run the Los Angeles Unified School District or the Chicago Public Schools for a year, among many other things.

For the first time, federal auditors are suggesting that some or all of the cash may have been stolen, not just mislaid in an accounting error. Stuart Bowen, special inspector general for Iraq reconstruction, an office created by Congress, said the missing $6.6 billion may be “the largest theft of funds in national history.”

The mystery is a growing embarrassment to the Pentagon and an irritant to Washington’s relations with Baghdad. 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.

It’s fair to say that Congress, which has already shelled out $61 billion of U.S. taxpayer money for similar reconstruction and development projects in Iraq, is none too thrilled either.

“Congress is not looking forward to having to spend billions of our money to make up for billions of their money that we can’t account for, and can’t seem to find,” said Rep. Henry A. Waxman, D-Calif., who presided over hearings on waste, fraud and abuse in Iraq six years ago when he headed the House Government Reform Committee.

Theft of such a staggering sum might seem unlikely, but U.S. officials aren’t ruling it out. Some U.S. contractors were accused of siphoning off tens of millions in kickbacks and graft during the post-invasion period, especially in its chaotic early days. But Iraqi officials were viewed as prime offenders.

The U.S. cash airlift was a desperation measure, organized when the Bush administration was eager to restore government services and a shattered economy to give Iraqis confidence that the new order would be a drastic improvement on Saddam Hussein’s Iraq.

The White House decided to use the money in the so-called Development Fund for Iraq, which was created by the Federal Reserve Bank of New York to hold money amassed during the years when Saddam’s regime was under crippling economic and trade sanctions.

The cash was carried by tractor-trailers from the fortresslike Federal Reserve currency repository in East Rutherford, N.J., to Andrews Air Force Base in Maryland, then flown to Baghdad. U.S. officials there stored the hoard in a basement vault at one of Saddam’s former palaces, and at U.S. military bases, and eventually distributed the money to Iraqi ministries and contractors.