WASHINGTON: For more than seven months, US 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.

The hijackers “left no paper trail,” FBI Director Robert S. Mueller III said in the text of a speech the FBI released Monday. “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.”

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 US 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 cell phones, coupled with hard-to-trace prepaid calling cards.

The FBI and other US intelligence agencies have come under intense scrutiny since Sept 11, and some analysts suggested that Mueller’s comments, made in a speech to the Commonwealth Club in San Francisco on April 19, may reflect an attempt to rationalize the intelligence community’s failure to pick up on any signs that an attack was imminent.

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.

Mueller acknowledged the tremendous challenges that US law enforcement faces in heading off another attack, and the nation has not done enough to stop it.—Dawn/LAT-WP News Service (c) Los Angeles Times.