WASHINGTON (CNN) -- A comment Defense Secretary Donald Rumsfeld made during a Christmas Eve address to U.S. troops in Baghdad has sparked new conspiracy theories about the terrorist attacks of September 11, 2001.

In the speech, Rumsfeld made a passing reference to United Airlines Flight 93, which crashed in Pennsylvania after passengers attempted to stop al Qaeda hijackers.

But in his remarks, Rumsfeld referred to the "the people who attacked the United States in New York, shot down the plane over Pennsylvania."

A Pentagon spokesman insisted that Rumsfeld simply misspoke, but Internet conspiracy theorists seized on the reference to the plane having been shot down.

"Was it a slip of the tongue? Was it an error? Or was it the truth, finally being dropped on the public more than three years after the tragedy" asked a posting on the Web site WorldNetDaily.com.

Some people remain skeptical of U.S. government statements that, despite a presidential authorization, no planes were shot down September 11, and rumors still circulate that a U.S. military plane shot the airliner down over Shanksville, Pennsylvania.

A Pentagon spokesman insists Rumsfeld has not changed his opinion that the plane crashed as the result of an onboard struggle between passengers and terrorists.

The independent panel charged with investigating the terrorist attacks concluded that the hijackers intentionally crashed Flight 93, apparently because they feared the passengers would overwhelm them.