By Gordon Corera
BBC security correspondent
The capture of alleged al-Qaeda computer expert Mohamed Naem Noor Khan by the Pakistani authorities in July brought with it an unprecedented haul of high-tech intelligence.
Speaking in London last month, America's homeland security chief Tom Ridge said the volume of potential information was "the largest we've ever seen - I mean potentially millions and millions of pages of information", and revealed that intelligence officers had yet to decipher it all.
Communications have always been an essential part of al-Qaeda's strategy, but the internet and email have become even more important in recent years.
They have provided the terror network with new possibilities - but, as the Khan case illustrates, fresh vulnerabilities too.
WHY THE WEB?
"The terrorists have fully exploited the modernisation of communications to their advantage," says Sajjan Gohel of the Asia-Pacific Foundation.
"The al-Qaeda ideology can be very well served on the internet. It is able to purport its agenda, goals and ideology probably better on the internet than any other means."
Having lost its base in Afghanistan, al-Qaeda is setting up on the net
As the core of the al-Qaeda leadership has been put under pressure, the organisation has been forced to evolve and become more decentralised - a structure to which the internet is perfectly suited.
The web offers a way for like-minded people located in different communities to interact - especially important when operatives may be isolated and having to lie low.
Denied a physical place to meet and organise, al-Qaeda has increasingly sought to create a virtual community through these chat rooms and websites to spread its propaganda, teaching and training.
"They lost their base in Afghanistan, they lost their training camps, they lost a government that allowed them do what they want within a country. Now they're surviving on internet to a large degree. It is really their new base," says terrorism expert Peter Bergen.
Those who watch the issue closely believe that it would be unlikely for al-Qaeda to use websites to communicate about the most sensitive matters, preferring to use it for general debate, the exchange of views and attempts to spread its thinking.
The propagation of a jihadist ideology has always been a central goal of al-Qaeda and the internet provides the broader jihadist movement with an effective vehicle to do this.
But it is not without liabilities, as it can prove a useful way of tracking down the network and closing in on cells.
Khan's laptop contained a great deal of information - including a huge number of names. Some reports claim that Khan was in fact working against al-Qaeda - sending messages out to the network and monitoring their response to track down members, although this has not been confirmed.
One of the most important weapons in Al-Qaeda's armoury is its ability to generate fear. Jihadist groups in general use videos and still images of attacks - and especially gruesome attacks like beheadings - as part of psychological warfare.
Sites may provide instructions on getting involved in Iraq
These videos and messages have the dual impact of spreading anxiety among enemies while simultaneously boosting morale for sympathisers and like minded groups.
As well as videos, messages purporting to be from al-Qaeda or al-Qaeda related groups also make frequent appearances on certain sites. These often involve warnings against countries coupled with the threat of imminent attacks.
Some of these are undoubtedly false, others probably real. And while some of the messages are directed internally, in a number of cases - the messages, like the images - are designed to scare.
Analysts believe that in some cases, messages are issued to set broad strategic objectives or target lists of places and people who should be subject to attack with the hope that independent cells, individuals and networks will then act on these in their own way.
They sometimes display a high degree of political awareness. One important paper entitled "Jihadi Iraq: Hopes and Dangers", spotted in Norway in December, looks at how members of the coalition occupying Iraq could be peeled off through terrorist attacks.
Spain is identified as presenting a particular opportunity given the general election that was then a few months off. It could, of course, be a coincidence that Madrid was then targeted just days before its election, but no one is sure.
There is less evidence of the internet being used by the Al Qaeda core for issuing specific orders or operational commands, or even for fundraising.
Email is used more often, but al-Qaeda operatives are trained to avoid detection, only using cyber-cafes briefly and using an email account for only one message before abandoning it.
Complex but publicly available encryption could also be used, but using this technology could draw attention - rendering regular message boards and groups a safer way of operating.
The 9/11 hijackers communicated through internet chat rooms, often using codenames and codewords for their plans. There is also some debate over a technique called steganography, which involves embedding secret messages within publicly available files and images on the internet, but there is little evidence so that al-Qaeda has used this.
Similarly there is some debate over whether messages from Osama Bin Laden and his deputy Ayman Al-Zawahiri contain any kind of message triggering groups around the world to attack. Again here the evidence is mixed.
As well as propaganda and ideological material, jihadist sites seem to be heavily used for practical training, important since the loss of Afghan training camps.
Sites like Al Battar contain hugely detailed information on how to kidnap VIPs, conduct surveillance and fire rocket propelled grenades.
Prior to the Madrid train bombings, one site had discussed Spain
Careful instructions were recently posted on one jihadist website on how to use mobile phones as detonators for explosives, as was used in Madrid.
Al-Qaeda's "Encyclopaedia of Jihad" which runs to thousands of pages has also been distributed over the internet.
The internet can also be a place on which terrorists can research information and find details of potential targets for attack, often known as 'cyberplanning'. Al Qaeda training manuals have emphasised the usefulness of publicly available information about its enemy. Huge amount of reconnaissance material and files have been found on captured computers.
Arguments also rage in chat rooms over what tactics and methods of violence are valid - whether hostage taking, beheading or the role of innocent - sometimes Muslim - casualties, in attacks.
The aftermath of the Russian school siege was interesting because many messages condemning what had happened were posted on sites which usually carry gruesome pictures and videos.
Recruitment, often through chat rooms, may take place, and there is some evidence that people have been given instructions on how to get involved in Iraq.
This summer, a new online publication seeking specifically to encourage women to take part in Jihad appeared.
Researchers have also noted a rise of participants in radical Islamic forums who write in English or using the Latin alphabet, possibly reflecting a rise of interest in Muslims in the West in these sites.
Many Western governments have tried to respond to the communications challenge, using numerous methods that include monitoring traffic on the net and exchanges in chat rooms, using "sniffers" - programs that check e-mails for suspicious material, and blocking access to certain websites which contain sensitive information, according to Gabriel Weimann of Haifa University, who has extensively researched the subject.
The posting of videos is an especially important sign to counterterrorism analysts as it suggests that a particular website must have access or be used by those who actually carry out acts of violence.
An elaborate game of cat and mouse is often played out as websites move URLs regularly, changing address to avoid being shut down by ISPs and hackers. The more organised tend to have mirror sites. Alneda.com which has been thought to be al-Qaeda's primary website, keeps moving, often by embedding itself within innocent websites.
"As the technology develops, the terrorists keep ahead of the pace as well," warns Sajjan Gohel of the Asia-Pacific Foundation.
Apart from the occasional video and audio tape, there is less evidence for Osama Bin Laden himself using high-tech methods. During the 1990s he did communicate through a satellite phone before the fact that the US could listen in was leaked.
Since then, analysts believe that he mainly communicates through human couriers.