United Nations ? Half the 4,500 companies that took part in the U.N. oil-for-food program in Iraq paid kickbacks or illegal surcharges and are being given a chance to respond to the accusations, two top investigators told The Associated Press.

The U.N.-backed probe is expected to release a major report in early September on the $64 billion operation and a final report in October on the companies involved in the purchase of Iraqi oil or sale of humanitarian goods under the program, the investigators said.

“We will report on the management and the corruption,” former U.S. Federal Reserve chairman Paul Volcker, who heads the investigation, said in an interview Monday.

Volcker said “the definitive list” of more than 4,500 private contractors involved in the program will include for the first time the entities behind so-called front companies.

Richard Goldstone, a former Yugoslav war crimes prosecutor and a member of Volcker’s Independent Inquiry Committee, said afterward that many contracts had side letters containing “evidence of kickbacks.”