WASHINGTON, Sept 20 (Reuters) - A senior U.S. lawmaker on Thursday made public a threat to subpoena the U.S. State Department for documents and the testimony of two of its officials as part of an investigation into corruption in Iraq.

The panel's investigation is part of increased oversight of the Republican Bush administration since the Democrats won control of both houses of the U.S. Congress last year, largely because of popular opposition to the war in Iraq.

In a letter to U.S. Secretary of State Condoleezza Rice, House of Representatives oversight committee Chairman Henry Waxman, a California Democrat, said her agency refused to turn over reports on the Iraqi Commission on Public Integrity prepared by the State Department's Office of Accountability and Transparency.

The letter, which was released by Waxman's office, said the State Department refused to allow interviews with two officials in its Office of Accountability and Transparency. Waxman had wanted to interview the officials ahead of a planned hearing on corruption in Iraq that has been put off one week to Sept. 27.

Waxman said the department did allow the panel to review the documents at the State Department and permitted a third official from that office to speak to committee staff members.

"If the State Department persists in refusing to provide the documents and arranging the interviews, the Committee will issue subpoenas for the documents and the officials' appearance at a deposition," Waxman said. "I would like to avoid the need for the use of this compulsory process."

A subpoena is a form of U.S. legal demand for information that is compulsory, although the State Department could refuse to comply and the matter could ultimately be decided in court.

Waxman also said he was worried that the State Department might "retroactively classify the two reports to protect the Department from the release of embarrassing information."

The State Department declined immediate comment on the matter.

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