From the NYT
http://www.nytimes.com/2005/08/09/politics/09intel.html?hp&ex=1123646400&en=8cdd39c879100274&ei=5094&partner=homepageBy DOUGLAS JEHL
Published: August 9, 2005
WASHINGTON, Aug. 8 - More than a year before the Sept. 11 attacks, a small, highly classified military intelligence unit identified Mohammed Atta and three other future hijackers as likely members of a cell of Al Qaeda operating in the United States, according to a former defense intelligence official and a Republican member of Congress.
In the summer of 2000, the military team, known as Able Danger, prepared a chart that included visa photographs of the four men and recommended to the military's Special Operations Command that the information be shared with the Federal Bureau of Investigation, the congressman, Representative Curt Weldon of Pennsylvania, and the former intelligence official said Monday.
The recommendation was rejected and the information was not shared, they said, apparently at least in part because Mr. Atta, and the others were in the United States on valid entry visas. Under American law, United States citizens and green-card holders may not be singled out in intelligence-collection operations by the military or intelligence agencies. That protection does not extend to visa holders, but Mr. Weldon and the former intelligence official said it might have reinforced a sense of discomfort common before Sept. 11 about sharing intelligence information with a law enforcement agency.
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The account is the first assertion that Mr. Atta, an Egyptian who became the lead hijacker in the plot, was identified by any American government agency as a potential threat before the Sept. 11 attacks. Among the 19 hijackers, only Khalid al-Mihdhar and Nawaf al-Hazmi had been identified as potential threats by the Central Intelligence Agency before the summer of 2000, and information about them was not provided to the F.B.I. until the spring of 2001.