WASHINGTON (AP) _ A federal commission investigating the Sept. 11 attacks voted Friday to issue a subpoena to the Pentagon for documents related to the activities of U.S. air defenses on the day of the terrorist hijackings.

``In several cases we were assured that all requested records had been produced but we then discovered, through investigation, that these assurances were mistaken and that records of importance to our investigation had not been produced,'' the National Commission on Terrorist Attacks Upon the United States said in a statement.

The 10-person, bipartisan panel said it raised its concerns with Defense Secretary Donald H. Rumsfeld, who pledged to help and ``has already taken strong steps to back up this pledge.''

The independent commission voted last month to issue a subpoena to the Federal Aviation Administration for documents pertaining to the investigation. The commission said the FAA subpoena would ``put other agencies on notice that our document requests must be taken as seriously as a subpoena.''

The panel, headed by former New Jersey Gov. Thomas H. Kean, a Republican, has also encountered resistance from the White House in its efforts to review sensitive intelligence documents.

The panel's statement did not address the White House dispute.

The commission has until May 27 to report on the events of Sept. 11, 2001 and issues of diplomacy, immigration, commercial aviation and the flow of assets to terror organizations.

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On the Net:

National Commission on Terrorist Attacks Upon the United States:

http://www.9-11commission.gov