The U.S.-backed investigation into alleged abuses of the United Nations' Oil for Food program in Iraq has already collected more than 20,000 files from Saddam Hussein's old regime and hired an American accounting firm to conduct the review.

Documents obtained by The Associated Press show the U.S.-backed, Iraqi-run Board of Supreme Audit selected the Ernst & Young firm this week to oversee the audit of the documents gathered from at least 16 former ministries of Saddam's government.

The U.S.-led Coalition Provisional Authority also is trying to head off a separate investigation launched by former Iraqi dissident Ahmad Chalabi, now an influential member of the Iraqi Governing Council, in hopes that a single, independent investigation will have more credibility.

Chalabi took an early lead in exposing alleged abuses by the U.N.-backed program and has been trying to force the coalition government to give him the $5 million in Iraqi funds set aside for the probe to pay for his effort. The move was strongly resisted by Paul Bremer III, who runs the governing Coalition Provisional Authority, or CPA.

The Chalabi-backed investigation started in mid-April, a month after Bremer announced the U.S.-backed one and made clear he would pay for only the probe. Chalabi's backers hired a different firm, KPMG, to do its audit, but they want Bremer's administration to pay the bill from the Iraqi funds it controls. The money comes from a fund of mostly seized Saddam assets and Iraqi oil sales.

"I must reiterate that CPA has approved only one investigation," Bremer wrote Chalabi this week. "The CPA will not authorize funding for other investigations into these allegations and any such investigation could undermine the process already under way."

The United Nations is conducting a third investigation. It named former Federal Reserve chairman Paul Volcker to lead a probe into whether U.N. officials helped siphon away money from the food for oil program.

Bremer's letter credited Chalabi for bringing "worldwide attention to corruption in the program" but demanded that his backers turn over to the U.S-backed investigation any documents or evidence it gathered independently.

"Bringing individuals involved to justice is important for the Iraqi people and remains a top CPA priority," Bremer wrote.

Chalabi and his backers have given no sign of backing down. This week, they issued a press release accusing Bremer of "stalling on the issue" and "undermining the Iraq Governing Council's efforts to get to the truth."

While Ernst & Young has just begun its work, the Iraqi Board of Supreme Audit already has collected relevant files from numerous former agencies of Saddam's deposed regime, according to documents obtained by AP.

The largest number of documents, 7,500 files, were seized from Saddam's former Oil Ministry, while more than 5,000 were taken from the Electricity Ministry, 3,682 from the Health Ministry and 2,500 from the Agriculture Ministry.

Records show that files also have been seized from a number of other ministries: irrigation, housing, industry, trade, religion, planning, youth and sports, higher education and transportation.

Allegations of corruption first surfaced in the Iraqi newspaper Al-Mada, which published a list of about 270 former government officials, activists, journalists and U.N. officials from more than 46 countries suspected of profiting from Iraqi oil sales under the program, which was designed to provided humanitarian relief to Iraqis after the first Gulf War in 1991.

Proceeds from the limited oil sales were supposed to help soften the blow of U.N. sanctions against Saddam on Iraqi women and children.

But the General Accounting Office, Congress' investigative arm, estimated in March that Saddam's government pocketed $5.7 billion by smuggling oil to its neighbors and $4.4 billion by extracting kickbacks on otherwise legitimate contracts.