In detailing his case against Iraq, the Secretary of State put on display signal intercepts, satellite photographs, recordings of private conversations and reports from captives and spies within the country.

The US intelligence community, most notably the Central Intelligence Agency and the National Security Agency, has always been fiercely protective of its sources and methods.

Jeffrey Richelson, an intelligence analyst in Washington, said: “There’s never been a case when the United States intelligence community has disclosed so much at one time.”

In interviews with The Times yesterday, former intelligence officials and independent experts detailed the scope and sophistication of the US high-tech spy network, how conversations are intercepted, and certain sites targeted by spy satellite cameras.

The most damning evidence produced by General Powell was conversations between Iraqi military officers, particularly one in which a colonel orders a captain not to mention the phrase “nerve agents” in wireless instructions.

Conversations conducted on land-line telephones, mobile telephones, military radios or even inside a room can be picked up by satellites, spy planes, unmanned drones, electronic bugs planted by agents, or permanent listening stations based, for example, hundreds of miles away in Turkey.

There are three types of US communication and intercept satellites over the Gulf region, up to ten in total, including CIA Orion 2 satellites and US Air Force Mercury and Jumpseat satellites. More than 100 yards in diameter, and with antennae the length of a football pitch, they have an eavesdropping scope extending as far east as India and Pakistan, into large parts of southern Russia and northern Africa, and the whole of Europe.

They can pick up microwaves emitted from cellular telephones or from landline calls which need to be retransmitted every 20 miles at microwave repeating stations. They can also pick up transmissions used by military radio transmitters using VHF, UHF or short wave frequencies. All these signals can also be intercepted by pilotless aircraft within 300 miles of Iraq, or “Rivet Joint” spy planes, converted 707 cargo aircraft. These are filled with digital listening equipment and linguists. They can eavesdrop from as far away as the skies above Saudi Arabia or Turkey.

Knowing that conversations with his senior command are at risk of interception, Saddam Hussein paid Chinese companies millions of dollars after the Gulf War to install underground fibre-optic communication cabling, which is fairly secure from aerial interception. However, it is possible for an agent on the ground to plant electronic bugs on this cabling.

The key to using this technology efficiently is being able to filter the billions of conversations taking place. This is done with constantly updated and complex computer software that singles out conversations using triggers, including key words, frequencies, tone of voice, telephone numbers and time and place of call.

Tim Brown, of Globalsecurity.org, said: “They have been intercepting conversations in Iraq for 12 years. They have got to know voices and patterns very well.” The intercepts are relayed to the National Security Agency in Fort Meade, Maryland, and are then pored over by analysts.

As a result, Mr Brown said, the Iraqi military now uses frequency scrambling radios. But US satellites can listen to thousands of frequencies in the same instant and piece back together a scrambled conversation. The Iraqis also train their officers not to use open channels or easily intercepted methods of communication.

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