By Katty Kay BBC Washington correspondent

US intelligence officials have admitted they failed to unearth any sort of paper trail leading to the 11 September attacks.

The hijackers did not use laptops and stored nothing on computer hard drives. They dressed and acted like Americans

In the most detailed account so far of the investigation, the head of the Federal Bureau of Investigation (FBI) said that after seven months of relentless work America had found no hard evidence mentioning any aspect to the attacks on New York and Washington.

FBI Director Robert Mueller said his agents had chased literally hundreds of thousands of leads and checked every record they could lay their hands on, from flight reservations to car rentals to bank accounts.

They have hunted through caves in Afghanistan and credit card bills in America but so far the very best of US intelligence has been thwarted by 19 al-Qaeda hijackers, revealing just how little America knows about the 11 September attacks.

New warning

Mr Mueller said the hijackers hid communications by using hundreds of different pay phones and cell phones, along with hard to trace pre-paid phone cards.

FBI officials say a similar attack may be plotted now

They did not use laptops and stored nothing on computer hard drives.

They made sure that all money sent to fund the operation was wired in small amounts to avoid detection.

They also dressed and acted like Americans and when four of them were stopped for speeding in the days before the 11 September, they kept calm, avoiding suspicion.

What investigators do know is that the attacks on Washington and New York were in the pipeline for as long as five years and so officials said, it was very very possible that a similar attack was being plotted right now.