Anti-terrorism intelligence monitors "electronic chatter" to see when attacks are imminent - but what exactly is it? Where does it come from and how is it monitored?
By Megan Lane
BBC News Online
It happened in the weeks before 11 September, 2001, in the run-up to the Bali bombing last October, and ahead of the suicide blasts in Riyadh - a surge in "electronic chatter", then sudden silence shortly before each attack.
The US knew an attack was planned in Saudi Arabia, but not the target
Now the United States is on heightened alert at home and in Saudi Arabia after intelligence agencies picked up "chatter" about new attacks against western targets. Both the US and UK have temporarily closed their embassies in the kingdom.
Similarly, British Airways has cancelled flights to and from Kenya amid fears of an attack on a UK plane, an alert said to have come from electronic eavesdropping on al-Qaeda suspects.
"Chatter" is the innocuous-sounding term for intercepted phone calls, e-mails and faxes between those suspected of plotting terror attacks.
Simply firing off an e-mail with the words "bomb" and "bin Laden" is unlikely to set alarm bells ringing, says Neil Robinson, of the Information Assurance Advisory Council. If, for instance, a journalist were to e-mail the cyber security expert to ask about surveillance and terror attacks, the message could be picked up by an automated system sifting for potentially suspicious key words.
Yet it is unlikely to be flagged up for the attention of a human analyst, for the system also checks e-mail accounts and IP addresses. Thus an exchange between the BBC's west London office and the Cambridge-based think tank would be of little interest to the intelligence services.
Eyes and ears
Not that the agencies involved - among them MI5, the Government Communications Headquarters (GCHQ) at Cheltenham, and the US's National Security Agency (NSA) - will discuss chatter and how it is monitored.
Mobile phones are among the easiest communications devices to scrutinize. When Army tanks rolled in to protect Heathrow against a possible missile attack last February, the GCHQ - which usually listens in on communications abroad - tuned into domestic airwaves to try to pick up any conversations of a hidden terror cell.
Intelligence indicated an attack on Heathrow was imminent
Mr Robinson says much of the "chatter" reported probably refers to scrutinisers noting a surge in the volume of traffic between suspects, rather than tapping into what is being discussed.
"They may track how long a call is made for; where a call is made from and to; the number of calls made or e-mails sent."
Others may track postings on newsgroups known to be used by those of interest to the authorities, be they May Day protesters or a group altogether more sinister.
"Terrorists the likes of al-Qaeda know they have to be very careful about how they communicate, and some - like Osama bin Laden himself - prefer to use non-electronic means. There were reports of communicating by horse-back in Afghanistan, sending messengers to deliver face-to-face communications."
For a phone call gave away the Karachi hide-out of Ramzi Binalshibh, the senior al-Qaeda suspect caught last September. It is thought a sample of his voice - recorded from an al-Jazeera interview - was fed into the NSA's computers; within days he made a satellite phone call, and the US had a location.
Eavesdropping led the US to Khalid Sheikh Mohammed
And an intercepted e-mail led to the arrest last March of another suspected bin Laden aide, Khalid Sheikh Mohammed. As "chatter" indicated he was planning further attacks, satellite tracking of his associates' communications threw up an e-mail with his address.
Intelligence gathered in this way is shared between American and British intelligence agencies, as well as those in Canada, New Zealand and Australia - information thought to be gathered through the shadowy surveillance network Echelon.
Phone calls, faxes and e-mails monitored, and suspects' computers hacked into
Key words include names, places, code words or subjects that might be of interest
Suspect content is passed to the relevant intelligence service for further analysis
As befits a covert system, there is much debate as to whether Echelon even exists - Mr Robinson is one of many who argues that there is simply too much communication zipping about to be monitored effectively.
Knowledge of Echelon's existence has come from the Australian and New Zealand governments, and the efforts of civil rights groups.
During the Cold War, it helped spies keep watch on opposing powers. Since then, the focus has switched to monitor threats to national security, hostile powers, and to help fight terrorism. To this end, the US operates listening posts at nine RAF bases in the UK, including Menwith Hill (pictured above) and Fairford.