WASHINGTON — Federal auditors conceded that $6.6 billion in cash sent to Iraq in cargo planes after the US-led invasion could have been stolen.

The money was part of a $12 billion haul that the George W. Bush administration sent to Iraq in the months following the fall of Saddam Hussein’s regime to pay for reconstruction and support government services, the Los Angeles Times reported Monday.

After repeated investigations into the missing funds since 2005, federal auditors in Washington are now suggesting the cash may have been stolen, not just mislaid in an accounting error. It could be “the largest theft of funds in national history,” according to Scott Bowen, special inspector general for Iraq reconstruction.

Pentagon officials have testified that reconstruction money was not closely monitored during the chaotic post-invasion period of 2003-2004. Millions of dollars was stuffed into sacks and loaded on pickup trucks before being delivered to contractors and government departments, they said.

In 2005, House Government Reform Committee investigators said there was “evidence of substantial waste, fraud and abuse in the actual spending and disbursement of the Iraqi funds.”

The United States’ inability to account for the cargo plane cash is causing tension with the Iraqi government, which is threatening to take the US to court.

Iraqi officials say the US was responsible for safeguarding the money — which came from Iraqi oil sales, seized Iraqi assets and the United Nations’ oil-for-food program — under a 2004 legal agreement it signed with Iraq.

“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,” Rep. Henry A. Waxman (D-Beverly Hills) told the Los Angeles Times.

The United States has spent $61 billion on reconstruction and development in Iraq since the US-led invasion.