Washington ? Piles of U.S. currency, hundreds of millions of dollars so far, are being found in Iraq, even though the country has been under economic sanctions for nearly 13 years.

Investigators — on the ground in Iraq and in the United States — are trying to track the money back to where it came from, a Herculean task, both officials and outside experts say.

The experts say there are plenty of possibilities, including oil and cash smuggling schemes, illegal trade deals, sham businesses and a web of middlemen located outside the country to conceal the true destination of the funds.

“Identifying a money trail can be very difficult to do,” said Jimmy Gurule, a law professor at the University of Notre Dame. “That’s why it is so essential that some documentation of financial records is discovered. Then investigators can go backward and trace the movements of the funds.”

Gurule recently served as the Treasury Department’s undersecretary for enforcement in charge of the government’s efforts to catch terrorists’ financiers. He left the job in early February.

Investigators also are looking into whether any of the cash found in Iraq was counterfeited.

In Baghdad, U.S. soldiers — trying to stop looting — discovered more than $600 million in tightly wrapped packets of new $100 bills hidden behind a false wall, U.S. military officials said Tuesday. The $100 bill is the most counterfeited U.S. note outside the United States.

In a neighborhood along the Tigris River, where senior Baath Party and Republican Guard officials lived, U.S. forces found some $656 million in U.S. currency, the Los Angeles Times reported last week.

It was not clear whether the find described by the military officials was the same as the one reported by the Times.

“When you discover such a huge amount of U.S. currency, you know you are not dealing with Boy Scouts,” said Robert Bonner, commissioner of the Bureau of Customs and Border Protection.

The Bush administration wants any genuine U.S. currency found in Iraq to be used to help the people of the country, Treasury officials said.

Separately, roughly $1.2 billion in illicit Iraqi assets have been recently uncovered abroad, said Treasury officials, who wouldn’t disclose details.

U.S. officials believe there is more Iraqi money that hasn’t been found, a major focus of investigators. A General Accounting Office report last year said that Iraq generated $6.6 billion in illegal revenue from oil smuggling and other schemes from 1997 to 2001.