More technology may not have stopped the attacks
Experts are warning of the risk of relying on technology to help spot terrorists before they attack.
Civil liberty groups add that intelligence agencies already have more than enough freedom to intercept communications and acquire records of electronic communications, and caution against handing over yet more powers to allay short-term fears.
Imposing restrictions on technologies that can be used to secure messages will do little to combat terrorism, they say, but could seriously erode personal privacy.
Hidden messages
The FBI has revealed few details of how last week's attacks were co-ordinated.
But, even so, it is clear that far from being a sophisticated operation using false identities, elaborate cover stories, uncrackable encryption and the highest of technology, the hijack assault was an extremely low tech mission.
The hijackers used their own names, public web terminals, frequent flier identifiers, and unencrypted e-mail messages to keep in touch.
"We are all focusing on this as a very hi-tech war, whereas the terrorists are using very low-tech means," said Brian Gladman, former technical director at Nato, and now an advisor to the net thinktank the Foundation for Information Policy Research (Fipr).
There were good reasons for keeping communications so simple, he said: "There's so little encryption going across the net that any that is used stands out like a sore thumb."
Reports suggest that the sophisticated technologies available to intelligence agencies mean that those planning attacks tend to favour low-key methods that are far harder to pick out and track. Many groups use messages passed by trusted messengers and word of mouth rather than electronic networks.
To avoid raising the suspicions of intelligence agencies, the groups are thought to fix the meaning of seemingly innocuous messages long before attacks take place. A message such as "See you in New York on the 11th" sounds innocent and its significance would only become apparent after the event.
Using porn
Before now, there has been speculation that Osama Bin Laden has hidden messages in pornographic images posted and swapped on Usenet, eBay and Amazon.
However, after analysing over two million images from eBay, Niels Provos and colleagues from the University of Michigan have said they found no evidence of hidden messages. Mr Provos and his colleagues are now extending their work to check more images.
Bruce Schneier, cryptography expert and founder of the Counterpane security consultancy, said banning encryption would do no good.
"Attempting to control encryption will not keep it out of the hands of the bad guys," he said.
"Good encryption helps more than it hurts," he said. "Any limitations on the use of encryption will make us less secure, and not more."
In the US, cyber-liberty groups, such as the Electronic Frontier Foundation (EFF) and the Electronic Privacy Information Center (Epic), have asked supporters to lobby politicians to stop them supporting the hastily drafted Anti-Terrorism Act. The EFF warned: "[The Act] would dramatically alter the civil liberties landscape through unnecessarily broad restrictions on free speech and privacy rights in the United States and abroad."
Many fear that an emergency session of EU leaders to he held on 21 September could lead to calls for similar restrictive legislation across European nations.
The EFF said that law enforcement agencies such as the FBI already had more than enough powers to tap communications and spy on citizens. It feared that extending these powers could mean routine surveillance of huge numbers of people.
"Everyone is outraged by the terrorist attacks, but that's no reason to suddenly abrogate our rights," said EFF legal counsel Deborah Pierce.