WASHINGTON - For more than seven months, U.S. authorities probing the Sept. 11 attacks have scoured everything from caves to credit cards in the expectation that they would ultimately discover how the 19 hijackers plotted their brazen scheme.
But the global search has produced virtually nothing in the way of hard evidence about the terrorists' planning, and authorities said Monday that they now face the growing realization that they may never know many key details.
That sobering conclusion underscores the skill and sophistication of the al Qaeda terror network in its ability to conceal its activities -- and the equally daunting difficulties that authorities face in heading off another attack, officials said.
''In our investigation, we have not uncovered a single piece of paper -- either here in the United States or in the treasure trove of information that has turned up in Afghanistan and elsewhere -- that mentioned any aspect of the Sept. 11 plot,'' said FBI Director Robert Mueller in the text of a speech the FBI released Monday.
His remarks offer the FBI's most comprehensive and detailed assessment to date of its investigation, remarkable as much for what investigators have not found as for what they have.
Mueller revealed that investigators believe the Sept. 11 plan may have been in the works for as long as five years, and that the hijackers used ''meticulous planning, extraordinary secrecy and extensive knowledge of how America works'' to conceal their scheme after entering the United States legally from the Middle East.
Investigators have found no computers, laptops, hard drives or other storage media that may have been used by the hijackers, who hid their communications by using hundreds of different pay phones and cellphones, coupled with hard-to-trace prepaid calling cards.
In executing wire transfers to fund the attacks, they were also careful to send money in small amounts, avoiding large cash transactions that would have triggered a government report, Mueller said, and added: ``The hijackers did all they could to stay below our radar.''
In hindsight, several episodes could conceivably have alerted authorities to a possible plot, including indicted al Qaeda member Zacarias Moussaoui's suspicious activities at a Minnesota flight school and the entry of two of the hijackers into the country even though they were on a CIA terrorist ``watch list.''
But even now, law enforcement officials say that while they have been able to reconstruct the movements of the hijackers in the months before the attacks -- all legal except for a few speeding tickets -- they have found no evidence of their actual plotting.
In a slew of attacks in Europe, Africa and the United States during the 1990s linked to al Qaeda and Osama bin Laden evidence would inevitably turn up after the fact revealing how the plans were laid, said terrorism expert Daniel Benjamin, a former National Security Council aide in the Clinton administration.
But Mueller's assessment about the Sept. 11 plot ''deepens the sense that these guys [in al Qaeda] have really taken a quantum leap in their ability to carry out an operation without all the traditional accouterments,'' he said.
Whether the hijackers never maintained a paper trail or managed to destroy the evidence before the attacks, Benjamin said, ``it's incredibly scary. It's premature to declare the record closed, and there may be something else out there that hasn't come out yet, but the fact that nothing has turned up is an indication of just what a skillful operation this was.''
William Wechsler, a former National Security Council official who specialized in al Qaeda financing, agreed that the Sept. 11 hijackers managed to avoid basic missteps that had flawed earlier operations.
''This shows we can't rely on the incompetence of terrorists to protect us,'' he said.
``The level of sophistication that they have shown in their ability to hide for so long among us . . . should make every American concerned that they could be doing the very same thing today.''