WASHINGTON Members of the independent commission that investigated the Sept. 11 terror attacks have called on Congress to determine whether the Pentagon withheld intelligence information showing that a secret American military unit had identified Mohamed Atta and three other hijackers as potential threats more than a year before the attacks.

The former commission members said the information, if true, could rewrite an important chapter of the history of the intelligence failures before Sept. 11, 2001.

"I think this is a big deal," said John Lehman, a Republican member of the commission who was navy secretary in the Reagan administration. "The issue is whether there was in fact surveillance before 9/11 of Atta and, if so, why weren't we told about it? Who made the decision not to brief the commission's staff or the commissioners?"

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Detailed accounts about the findings of the secret operation, known as Able Danger, were offered this week by Representative Curt Weldon, the Pennsylvania Republican who is vice chairman of the House Armed Services Committee, and by a former defense intelligence official.

An outspoken member of Congress on military and intelligence questions, Weldon, a champion of data mining efforts like Able Danger, has helped arrange interviews for reporters with the former military intelligence official. The official insisted on anonymity, saying he did not want to jeopardize political support for future data mining in the military.

The official said in an interview Monday that the Able Danger team was created in 1999 under a directive signed by General Hugh Shelton, chairman of the Joint Chiefs of Staff, to assemble information about Qaeda networks around the world.

He said that by the middle of 2000 the operation had identified Atta and three of the other future hijackers as members of an American-based cell and that the information was presented that summer in a chart to the Pentagon's Special Operations Command headquarters in Tampa, Florida.