Four years after the nation's deadliest terror attack, evidence is accumulating that a super-secret Pentagon intelligence unit identified the organizer of the Sept. 11 hijackings, Mohamed Atta, as an Al Qaeda operative months before he entered the U.S. Had the FBI been alerted to what the Pentagon purportedly knew in early 2000, Atta's name could have been put on a list that would have tagged him as someone to be watched the moment he stepped off a plane in Newark, N.J., in June of that year. Physical and electronic surveillance of Atta, who lived openly in Florida for more than a year, and who acquired a driver's license and even an FAA pilot's license in his true name, might well have made it possible for the FBI to expose the Sept. 11 plot before the fact. Anthony Shaffer, a civilian Pentagon employee, says he was asked in the summer of 2000 by a Navy captain, Scott Phillpott, to arrange a meeting between the FBI and representatives of the Pentagon intelligence program, code-named Able/Danger. But he said the meeting was canceled after Pentagon lawyers concluded that information on suspected Al Qaeda operatives with ties to the U.S. might violate Pentagon prohibitions on retaining information on U.S. persons," a term that includes U.S. citizens and permanent resident aliens. Asked by Sen. Arlen Specter (R-Pa.), chairman of the Senate Judiciary Committee, at a hearing last week whether Atta...was a "U.S. person," a senior Pentagon official answered, "No, he was not."