When security-cleared visitors were taken into the national security agency in Fort Meade, Maryland, their hosts, it is reported, used to play them an audio tape of Osama bin Laden talking to his mother. His satellite phone call had been intercepted by the largest and most powerful spy agency on earth.

The smug message to outsiders was clear: if we can listen to America's most wanted man making small talk with his family then, believe us, he can't use the bathroom without Washington knowing.

But the NSA has probably stopped showing off like that, because this story demonstrates the US problem: it and its western allies possess a unique array of surveillance technology which became ultimately pointless. And the west's great intelligence failure will outclass Pearl Harbour in the history books.

Stella Rimington, a former head of MI5, has observed this week in her memoirs that the purpose of counter-terrorism is not to take snapshots of their enemies' activities, or even to successfully trace the perpetrators, but to thwart attacks.

British ministers seemed to be attracted yesterday to the ideas of identity cards and more surveillance of ordinary citizens. But to stop terrorist attacks, the holy grail is something else: intelligence of the enemy's actual intentions.

US intelligence has a budget approaching Dollars 30bn (pounds 21bn) - roughly the size of the GDP of Kuwait. It employs more than 100,000 people, and owns vast arrays of hardware.

At the centre is George Tenet. He is the director of central intelligence and the man who coordinates the various US spy agencies. He has huge resources. The espionage agency, the CIA, employs more than 16,000 people around the world.

The FBI is one of the largest law enforcement agencies ever; with annual funding of about Dollars 3bn, it has 11,400 special agents and more than 16,400 other employees in 55 American cities and abroad. The FBI spends one-sixth of this budget on intelligence gathering alone.

But it is on technology where budgets - quite literally - go through the stratosphere. The existence of the national reconnaissance office (NRO) was declassified only in 1992. With its Dollars 6.2bn annual budget, its mission is to run spy satellites - "spaceborne assets needed to enable US global information superiority".

The national security agency collects foreign signals intelligence (SIGINT) and operates in conjunction with the NRO.

The NSA's 21,000 employees - including the world's largest collection of linguists and mathematicians - are based at Fort Meade, with the rest scattered overseas.

Nine other agencies, ranging from army intelligence (budget Dollars 1bn) through to the departments of treasury, energy, transportation (more than Dollars 1bn) and the national imagery and mapping agency (Dollars 1.2bn) are also involved in intelligence gathering. Add into the equation intelligence work by non-intelligence agencies and the budget exceeds Dollars 27bn.

Interception

The majority of these formidable systems have remained targeted on traditional forms of possible attack by hostile nation states - especially those that might have weapons of mass destruction.

Their efforts are governed by the vast array of technology at their disposal:

* Listening stations: The main allied station in the Middle East is now the British station, Ayios Nikolaos, in eastern Cyprus. Satellite interception antennae are directed at Arab, Turkish and international communications satellites. A listening station in China, run by the German BND intelligence service, stayed operating even after the Tiananmen Square massacre. If it is still operating in the Pamir mountains, it would be the west's most important electronic surveillance asset close to Afghanistan.

* Space surveillance: Satellites orbit the earth every 90 minutes. Some, known as Keyhole, take photographs; others, in the Lacrosse and Vega series, use radar to obtain images through cloud and at night.

All relay their information immediately to Washington.These satellites can obtain pictures of training camps, using infra-red or heat photography to determine whether camps are occupied. Modern commercial imaging satellites can detect one-metre objects. Spy satellites are about ten times more powerful. But only four of these hugely expensive satellites, which are easy to spot, may be in orbit at once.

* Listening satellites: Another network of between eight and 12 satellites intercept signals from the earth's surface. Menwith Hill, in Yorkshire, is the largest electronic surveillance field station in the world. It directs and controls a constellation of listening satellites now directed mainly at the Middle East. The dominant feature of the station, which employs dozens of Arabic, Farsi and Hebrew linguists, is an array of 29 giant, white golfball-like radomes, each of which hide a satellite-tracking antenna.

Launched into high fixed orbits about 40,000km above the earth, secret SIGINT satellites called Vortex, Magnum, Mercury and Orion unfurl listening dishes more than 50 metres in diameter. These satellites can intercept and relay low-powered radio signals, including mobile phones. They can also discriminate between individual operators and radio equipment.

* Spy planes: Under the codename "papermate", the Royal Australian Air Force sends spy planes over Pakistan to detect nuclear preparations. These missions, which have been mounted from the US Indian Ocean base at Diego Garcia, could be extended to Afghanistan if Pakistan or Iran were willing to cooperate. A sophisticated new system, called Classic Story, can identify individual voices and could flash a warning if a wanted person was heard.

* Echelon: This controversial global monitoring system targets international civil communications channels passing through satellites. At Morwenstow, Cornwall, millions of satellite phone calls, faxes, e-mails and data links are monitored every hour. Information is relayed automatically from Britain to the US. It is highly dependent on sophisticated computer filtering software, called dictionaries, which hold thousands of key words or other templates for selecting messages of interest.

* Internet: Dictionary-type systems also monitor the internet. NSA's taps into internet cable are among the spy agency's most sensitive secrets, because they scoop up huge quantities of messages sent by US citizens. The NSA has had to construct databanks capable of holding "petabytes" of data - up to 1m times bigger than an average PC.

* Submarines: The US Navy's special tapping submarine, USS Parche, won special commendations for its work throughout the 1990s. In 2004, the Parche will be replaced by a giant new tapping submarine, the USS Jimmy Carter, which is currently being refitted at a cost of more than Dollars 400m. Once in service, the Jimmy Carter will be able to carry large surveillance packages the size of a small bus on to the seabed. It will be equipped to tap modern optical fibre cables

* Embassies: Most American and many allied embassies and consulates are equipped with "special collection" equipment. For many years, the US embassy in Moscow tracked conversations Soviet leaders had from their limousines. Special monitoring equipment can be used to monitor and reconstruct emissions from unprotected computer screens.

Yet this array of technical hardware goes hand in hand with a lack of human intelligence - almost a prerequisite for the uncovering of terrorists' intentions.

Shortly before this week's atrocities in the US, someone who described himself as a former CIA operative wrote a long, bitter account from Peshawar published in Atlantic Monthly. His lament echoed other intelligence men before him. Reuel Marc Gerecht said that the CIA and FBI claims were a myth that they were clandestinely "picking apart" Bin Laden's organisation "limb by limb".

He said: "Westerners cannot visit the cinder-block, mud-brick side of the Muslim world - whence Bin Laden's foot soldiers mostly come - without announcing who they are. No case officer stationed in Pakistan can penetrate either the Afghan communities in Peshawar or the North West Frontier's numerous religious schools, which feed manpower and ideas to Bin Laden and the Taliban, and seriously expect to gather useful information about radical Islamic terrorism, let alone recruit foreign agents.

By contrast, a priority for the British and, indeed, most other agencies in engaged in "humint", or human intelligence, is long-term penetration.

The dangerous and subtle task of infiltrating, or recruiting informers, in terrorist groups has borne fruit in Northern Ireland where MI5, the RUC and the army's covert group, the force research unit, have penetrated both republican and loyalist paramilitary groups. British soldiers of Irish origin were persuaded to leave the army and spend several years ingratiating themselves with the IRA.

The Israeli intelligence services have also succeeded in recruiting informers among Palestinian groups in the occupied territories and refugee camps, as well as among the Palestinian population in Jordan and Syria.

Treachery

So it is not impossible to use "lo-tech" infiltration. One reason why the US became addicted to technology instead was the fear of double-agents and treachery - the Soviet-era molehunts left a long shadow of distrust over human beings. It was thought machines could not lie.

Another reason was given by Mr Gerecht. "Long-term seeding operations simply didn't occur," he said. He quoted one Middle East CIA man describing non-official covert placements: "NOCs haven't really changed at all since the cold ear We're still a group of fake businessmen who live in big houses overseas. We don't go to mosques and pray." The reason? "Operations that include diarrhoea as a way of life don't happen."

As if to compound the west's flawed commitment to technology at the expense of potentially crucial "lo-tech" infiltration, at the other end of the earth things are very different.

When Jamal Ahmed al-Fadl was told to deliver two letters from Pakistan to Cairo he was given simple instructions, as well as a false passport. Shave off your beard, wear western clothes, carry cigarettes and make sure you smell of cologne. The perfume, he was told, would convince Egyptian airport officials that here was a man who liked women.

Acting this way against US intelligence, Bin Laden and his followers look decidedly lo-tech. Yet an operation of some magnitude, involving more than 50 people and taking up to two years in the making, evaded the vast intelligence resources of the greatest power on earth. Al-Fadl, 38, from Sudan, was giving his testimony earlier this year in the southern district court of New York, a few blocks away from the World Trade Centre. As one of very few defectors from al-Qaida, the Islamic revolutionary group formed in Afghanistan 12 years ago by Bin Laden, Al-Fadl's evidence cast a fascinating insight into the complex, by turns primitive and sophisticated network, that Bin Laden and his fellow mojahedins spawned. He was a witness in the trial of four men since found guilty of taking part in the bombing of the American embassies in Tanzania and Kenya in 1998.

Swearing to obey

Al-Fadl, who left Sudan for the US in 1986, was recruited by the emir of the Farouq Mosque in New York, who reported to a headquarters in Pakistan. As one of the first volunteers to join al-Qaida, he saw how the charismatic Bin Laden - sometimes called "the director" - enjoyed the confidence of militants from across the Arab and Muslim world.

On the ruling shura council there were members from Yemen, Saudi Arabia, Nigeria, Iraq, Oman and Libya. Members, who signed a contract, or bayat , swearing to obey orders and be ready at all times, were recruited, often at senior levels, in fundamentalist groups across the Middle East.

Fighters and money were given to groups in Chechnya, Azerbaijan, the Phillipines, Eritrea, Somalia and Tajikistan. A London office was opened in 1994 and one member had a bank account at Barclays. Other accounts were held in Dubai, Malaysia and Hong Kong.

After the Russian withdrawal in 1989 and Bin Laden's relocation to the Sudan, al-Qaida owned farms, import and export businesses, and a construction company. But the real object was to export revolution, to set up true Islamic governments and overthrow the rich and corrupt regimes of the Gulf.

With such a wide network of contacts, with trusted members such as Al-Fadl always on the move, carrying coded messages and money, it is easy to see how difficult it was for the might of western intelligence to keep track of this hydra-headed phenomena.

There was, from the start, a sort of structure. Beneath the shura council were four committees. The military committee acquired and exported arms and ran an electronics workshop. Finance organised travel arrangements, false documents and salaries. There was a newsletter produced by the media committee and a fatwa committee which delivered judgments on issues of the day.

On two occasions the military committee shipped consignments of AK47s from Sudan to the outlawed jihad movement in Egypt. There was a simple solution - as simple as boarding an internal US flight carrying a penknife. Fifty camels were bought at a local market at Umduhrman near Khartoum and the guns were trudged across the open desert.

So important messages were often delivered face to face. In Sudan the intelligence service provided al-Qaida with a radio system. Bin Laden had an Dollars 80,000 portable INMARSAT phone from Germany which transmits and receives calls over satellites owned by the International Maritime Satellite Organisation.

US intelligence officials say Bin Laden is well aware they can eavesdrop on his international communications but he does not care. When Bin Laden wants to make sure his communications are secure, one technique is by sending encrypted messages through the internet.

Another informant who gave evidence at the embassy bombing trial was L'Houssaine Kherchtou, a Moroccan, who described how he was trained in reconnaissance techniques, including using tiny cameras discreetly "without using the camera straight in your eyes".

Kherchtou, who was recruited at an Islamic cultural centre in Milan, was also sent to Kenya to train to be a pilot. Although it never came to pass, he was told he would be Bin Laden's personal pilot. Trained pilots were clearly not readily on hand for Bin Laden.

It was clear from the trial evidence that the members of al-Qaida shared strong, personal bonds, even if there was resent ment that the many Egyptians in the group were paid more than the others.

There was a keen awareness of the possibility of infiltrators and informers; people who turned up in Sudan and later Afghanistan were rigorously vetted.

That, too, may explain why the US authorities, with all their fancy technology and intelligence input from their allies in the Middle East - Egypt, Morocco, Jordan, Israel and Saudi Arabia - were unaware of the 50 conspirators on American soil plotting a terrible outrage.

In Northern Ireland the IRA and Real IRA have relied on "cleanskins" - people with no past record, unknown to the security or law enforcement agencies - to organise terrorist attacks. This was also clearly the case with those involved in the attacks in the US.

Duncan Campbell is the investigative journalist and not the Guardian's Los Angeles correspondent of the same name

Science v cunning

The hi-tech US

* Central Intelligence Agency (espionage): Dollars 3.1bn (pounds 2.1bn) budget

* Federal Bureau of Investigation (investigators): Dollars 3bn budget

* National Security Agency (radio eavesdropping): Dollars 3.6bn budget

* National Reconnaissance Office (spy satellites): Dollars 6.2bn budget Intelligence methods

* Satellite cameras

* Spy planes

* Submarines

* Code-breaking

* Radio intercepts

* Internet intercepts

* Phone and fax intercepts

The low-tech terrorists

* Osama bin Laden: Dollars 205m estimated net wealth plus business enterprises and wealthy Gulf backers

Counter-intelligence methods

* Loose-knit network without even a cell structure across dozens of countries

* Not easily infiltrated

* Important communications in internet code, or by hand

* Adapt simple solutions

* Suicide bombers need no escape route

* Take the long view; laborious training

* Ambitious plans unimaginable to opponents