UNITED NATIONS — The office of the senior U.N. official in charge of the scandal-plagued Iraqi oil-for-food (search) program has sent letters to companies involved in the program telling them they should not hand over any documents or information without first clearing it with the United Nations.

According to the letters obtained by Fox News, the companies "should retain and safeguard" any documents related to the program and should provide them to U.N. officials upon request.  The letters came from the office of Undersecretary-General Benon V. Sevan (search), though aides signed the letters on his behalf.

One of the letters was sent to a company called Cotecna Inspection S.A. (search), which for five years had the job of authenticating all goods being shipped into Iraq under the oil-for-food program.

It's also the company that once employed the son of U.N. Secretary-General Kofi Annan (search).  Annan has said his son Kojo stopped working for the company before the Cotecna contract was awarded.

The second letter, dated April 27, was provided to Fox News with the company name hidden.  The source who provided the letter said it was one of the hundreds of companies authorized to do business with the oil-for-food program.

Both letters — as well as a third one made public earlier this week to Saybolt Corp. (search), an inspection agency hired by the United Nations to monitor the loading of Iraqi oil — remind the companies of their contractual confidentiality agreements.  For example, the April 2 Cotecna letter says all documents and data "shall be the property of the United Nations, shall be treated as confidential and shall be delivered only to United Nations authorized officials."

All three of these letters came from Sevan, who ran the oil-for-food program and who is accused of personally profiting $3.5 million through alleged illegal oil transactions.

Lawmakers on Capitol Hill expressed concern about the potential conflict of interest, saying that U.N. officials who are themselves being investigated appear to be trying to control the information investigators can access.

"We view our role in many cases as a catalyst.  Ultimately, the truth has to be known and the U.N. has to disclose it," said Rep. Christopher Shays (search), R.-Conn.   "We can't ignore the serious allegations of malfeasance. The U.N. is under an ominous cloud."

Sen. John Ensign (search), R-Nev., said the most scandalous actions may still be to come.

"We're afraid there could be a major cover-up that could (dwarf) the original scandal," Ensign told Fox News.   "We all know in America the cover-up can be worse than the crime and we certainly don't want this covered up ...   The United Nations certainly shouldn't be above the law."

U.N. officials said the letters are standard procedure to remind companies of their contractual obligations.

But a senior congressional aide involved in the investigation said that the longer the United Nations clings to these confidentiality agreements, the more it will suffer potentially irreparable credibility problems.

Sevan ran the program for seven years and is retiring on May 31, but U.N. officials said he would remain available for the investigation.

"Benon has stated quite clearly that he is innocent," Annan said last month.  "He has indicated he will cooperate, as I expect all other staff members to cooperate."

The U.N. chief declared that he was "very keen" for the three-member panel led by former U.S. Federal Reserve chairman Paul Volcker (search) to report "as soon as possible."  And he promised that any U.N. official found guilty of accepting bribes or kickbacks would be dealt with "very severely."

Sevan is currently out of the United States and has declined requests to speak to Fox News about his role in the program.  He has maintained in the past that he did nothing wrong, and would cooperate fully with the Volcker investigation.

The Volcker panel doesn't have subpoena authority and will rely on voluntary cooperation from governments, U.N. staff, members of Saddam's former government and current Iraqi leaders.  They claim they have evidence that dozens of people, including top U.N. officials, took kickbacks from the $67 billion oil-for-food program.

John Ruggie (search), a former U.N. assistant secretary general, told Fox News on Wednesday that he did not see the letters as a tactic to prevent investigators from getting needed information but rather as a way of conforming to the United Nation's general bureaucracy.

"The letter did not state that no documents will be released.  The letter stated 'check with us,'" Ruggie said. "They will be turned over to Paul Volker and that's where they belong ...  Volker owns every document that's relevant to the oil-for-food program."