Mr. Bowen kept the discovery and his investigation of the cash-filled bunker in Lebanon, which his office code-named Brick Tracker, secret. He has never publicly discussed it until now, and his frustration that neither he nor his investigators can fully account for the missing money was evident in a series of interviews. “Billions of dollars have been taken out of Iraq over the last 10 years illegally,” he said. “In this investigation, we thought we were on the track for some of that lost money. It’s disappointing to me personally that we were unable to close this case, for reasons beyond our control.”

He is equally frustrated that the Bush administration, apart from his office, never investigated reports that huge amounts of money had disappeared, and that after his investigators found out about the bunker, the Obama administration did not pursue that lead, either. Mr. Bowen said his investigators briefed the C.I.A. and the F.B.I. on what they found. But Mr. Bowen added that he believed one reason American officials had not gone after it was “because it was Iraqi money stolen by Iraqis.”

Spokesmen for the F.B.I. and C.I.A. declined to comment for this article.

The Iraqi government has also not tried to retrieve the money, and has kept information about the Lebanese bunker secret. Mr. Bowen said that he talked to Prime Minister Nuri Kamal al-Maliki about the missing money and his discovery of the bunker and that Mr. Maliki never took any action, while expressing anger at the way the United States had handled the airlifted cash.

The money so assiduously carried to Iraq from a vast facility in New Jersey operated by the Federal Reserve Bank of New York came from the Development Fund of Iraq, which was created by a United Nations resolution in May 2003 to hold Iraqi oil revenue. The fund was to be used in Iraq’s reconstruction, and the United Nations resolution called for the creation of a monitoring board to make sure the United States-led Coalition Provisional Authority, which governed Iraq in 2003 and the first half of 2004 and ordered the cash flights, used the money properly for the benefit of the Iraqi people.

For the C.P.A., an advantage of using the cash from the Development Fund instead of money appropriated by Congress for Iraq was that there were not a lot of rules governing its use, and no federal regulations or congressional oversight of what happened to it. It was Iraqi money, not anything from American taxpayers.