US oil refiner Valero Energy has become the third US company to receive a subpoena seeking information about its role in Iraq's oil-for-food programme.
Oil giants Exxon Mobil and ChevronTexaco confirmed on Friday that they had also been subpoenaed, Reuters news agency reported.
All three subpoenas came from the US Attorney's office for the Southern District of New York.
All three firms have said they intend to co-operate fully with the inquiry.
A subpoena is a request for documents to aid an investigation. No charges have been brought.
Valero spokeswoman Mary Rose Brown said the request centred on the refiner's purchases of Iraqi oil under the UN-administered oil for food programme between 1995 and 2003.
Exxon Mobil, ChevronTexaco and Valero were the three biggest buyers of Iraqi oil during the years of the oil-for-food programme.
The programme is the subject of separate investigations by the UN, the US Congress and the Iraqi government following allegations of corruption.
UN sanctions prohibited trade with Iraq after the 1991 Gulf war to evict Saddam Hussein's troops from Kuwait.
The oil-for-food programme was introduced to permit exports of Iraqi oil provided the money was used for food and medicines.
It was wound up in November 2003, after the US-led coalition had ousted Saddam Hussein's regime in Iraq, although oil revenue remained under international control.
Iraq's oil revenue will still be monitored by the World Bank and IMF, through the International Advisory and Monitoring Board, after Iraq's interim government takes power on 30 June.
However, the new government will be in charge of spending the money.