2pm: Saddam is spotted. 2.48pm: pilots get their orders. 3pm: 60ft crater at target

High above Iraq's western desert on Monday afternoon a US B-1 stealth bomber was refuelling in mid-air on its way to a scheduled series of air strikes when it received urgent orders. They came in the form of a new set of target coordinates, but an air traffic controller made it clear what they meant: kill Saddam Hussein.

"Those were the words that were used - 'This is the big one'," Lieutenant Colonel Fred Swan, the bomber's weapons officer recalled in a telephone interview with Pentagon reporters yesterday.

As far as Col Swan was concerned it was business as usual. "I did not know who was there. I really didn't care," he said. "We've got to get the bombs on target. We've got 10 minutes to do it. We've got to make a lot of things happen to make that happen. So you just fall totally into execute mode and kill the target."

It is still not clear whether this particular target was killed or not. The British say not, claiming that Saddam left the restaurant minutes before the bombing. But the Pentagon remained non-committal yesterday. What is clear is that 12 minutes after that radio conversation, at about 3pm Baghdad time, Col Swan's bombs left a 20-metre crater in the ground where a restaurant once stood in capital's prosperous Mansour district.

The hole was made by four bombs in what the air force likes to call a "package". A couple of 2,000lb specially hardened bombs were dropped from 6,000 metres and directed to the target by satellite guidance. They crushed the restaurant and at least one building next door before penetrating deep below, where it was believed there was a series of bunkers used by Iraqi intelligence.

Right behind those "bunker-busters" were two more bombs, just as big, but set on a 25-millisecond fuse, so that they would detonate underground. The idea was to kill as many people as possible in the bunker but do as little damage as possible above ground.

It sounds surgical in theory. In practice, it still leaves a mess. Television pictures of weeping local residents suggested there had been civilian casualties.

Strike

Whether Saddam, his sons and Ba'ath party aides were killed too, depends largely on what happened in the hour before Col Swan received his orders. It took only that time, the Pentagon has claimed, from the moment the target was positively identified to the call going out to the B-1 bomber from US Central Command.

When the CIA thought it had a fix on Saddam on March 19, it took more than four hours for President Bush to make the decision to strike, for the cruise missiles to be reprogrammed and for them to reach their target, in the surprise attack that began the war.

On that occasion and on Monday, it appears that a joint effort by CIA agents and US special forces were responsible for tracking down Saddam, in an operation that may herald a new era in covert warfare that has been promoted by the US defence secretary, Donald Rumsfeld.

According to a former special forces soldier who is in contact with officers involved in current operations, one of the units involved in the hunt was Grey Fox, a clandestine army intelligence-gathering unit specially trained in tracking down targets in enemy territory.

"It's their technical expertise that sets them apart. They are trained to operate by themselves, in parties of one, in an urban environment," he said. "They have the look, and they have the language, and they go in long before it starts."

Grey Fox, otherwise known as the Intelligence Support Activity (ISA) had its roots in the 1979 Iran hostage crisis which led the army to believe it needed its own undercover espionage unit, rather than having to rely on the CIA.

The unit's skills at getting close to its target and to intercept electronic signals were used against Hizbullah in Lebanon, war crimes suspects in Bosnia, and most successfully in the hunt for Pablo Escobar, the elusive head of the Medellin drug cartel hunted down by Colombian troops with ISA help in December 1993.

The leading role of clandestine special forces units such as Grey Fox in Iraq is doubly significant because as far as Mr Rumsfeld is concerned, they are the future. A policy paper he commissioned last summer advocated the creation a new organisation combining espionage and military force, to pursue terrorists and the leaders of "rogue states".

Grey Fox is unlikely to have been the only American unit inside Baghdad in the past fortnight. There are reported to be Delta Force commandos in the city, together with CIA teams, working in much closer coordination with the military than ever before.

One or more of these units acted as "forward air controllers" near the restaurant, helping identify the target and confirm that it had been hit. But before that Saddam had been tracked for several days. According to reports in the US yesterday, CIA paramilitaries and Delta Force soldiers had been close to killing him three or four times in the past week.

The hunt has taken several paths. It is almost certain that CIA and Grey Fox approached members of the regime in an attempt to persuade them to change sides. The other path involved electronic surveillance, a skill in which Grey Fox teams excel. Special forces have attempted to tap into the variety of means Saddam uses to communicate with his lieutenants.

On March 19, they are said to have intercepted conversations on underground fibre-optic cables used by the leadership. They can also listen into conversations on mobile phones, a rarity in Baghdad.

"If there is someone using a cell phone it's more likely to be one of the top level leaders than necessarily an ordinary Iraqi," said Kenneth Pollack, a former CIA analyst and expert on the Baghdad regime.

On Monday, however, some outdated British technology was involved. Saddam's hunters zeroed in on an old communications system made by the British company Racal, which the Iraqi leadership bought during the Iran-Iraq war for sending encrypted communications among themselves.

At the time, the Racal Jaguar V was a sophisticated secure combat radio system, equipped with two main security mechanisms: encryption to scramble message content and frequency-hopping at the rate of up to 200 times a second to make eavesdropping difficult and defeat enemy electronic jamming measures.

According to the New York Post yesterday, the British cracked the encryption code for the radios some time ago and passed the information to the Americans.

Steven Aftergood, a senior intelligence technology researcher at the Federation of American Scientists, said: "If it was really a 20-year-old system, then one has to assume that the encryption is obsolete, that it has been penetrated."

Other military analysts said it was just as likely that the British government would only have granted Racal export licences for the radios if it had already been satisfied it could break the encryption.

Ruper Pengelly, the technical editor of Jane's, said: "All of these exports had to be approved by the government, particularly encryption. It was generally understood that only encryption up to a certain level would be licensed for export, so if they allowed it out of the country it was a safe bet that it was crackable, although that is something they could never say publicly."

The first consignment of Jaguar radios to Iraq was in 1985, says Kenneth Timmerman, a journalist specialising in the Middle East and arms sale networks. Four years later, the British government licensed the sale of 13 Jaguar radios at a cost of $360,000 (£225,000). In the same year, according to Timmerman, Saddam bought 2,000 Jaguar kits worth $48m, and $4m of encryption technology.

That purchase 20 years ago appears to have led Saddam's hunters to their target on Monday afternoon.