FBI agents are searching the internet for clues left behind by the hijackers who slammed fuel-laden planes into the World Trade Center and the Pentagon.
The Federal Bureau of Investigation (FBI) has deployed a special unit, the "Computer Analysis Response Team" to conduct the web-based operation aimed at tracking down the terrorists behind the September 11 attacks.
"They are able to retrieve the information across the hard drive of computers used by the terrorists, whether it be web surfing, email, instant messengers, chat rooms, providers," said Curt Bryson, whose company trains federal agents in net-tracking techniques.
The FBI unit is largely concentrating on recovering any conversations the hijackers might have had, according to Mr Bryson.
To do this, agents use a system known as "Carnivore", which works through software that makes it possible to monitor and even break into the flow of electronic mail.
Since investigations started, the FBI has searched internet-cafes, public libraries equipped with computers the hijackers could have used, and hotels where they stayed.
The US daily Sun Sentinel reported that the FBI also examined hundreds of computers from libraries in Florida, where the hijackers lived for several months before the attacks.
Only days after launching the probe, the FBI announced the arrest in San Diego of a man identified as Omer Bakarbashat, 26, who is suspected of teaching the terrorists how to use the internet.
They also found out that the hijackers sought information about crop-dusting planes.