The terrorists knew that the three residential compounds in Riyadh were among the most popular with Westerners working in Saudi Arabia and were close enough together to enable simultaneous attacks.

It took the bombers less than 15 minutes to fight their way inside the complexes, all of which had the latest security, and to detonate their explosives next to villas and blocks of flats where families slept.

They knew the layout of each complex, some the size of an English village, and synchronised the assaults to ensure that security guards did not have time to warn neighbours or alert the authorities.

Mary Brown, a London-born nurse, had glanced at her watch at 11.22pm when she first heard machinegun-fire reverberating around the al-Jadawel complex. She ran to a back room with her American husband. Two minutes later, she said, every window in their villa was blown out and doors lifted from their hinges by an ear-splitting blast.

Two miles away, the clock inside the wrecked meeting hall in the al-Hamra village had stopped at 11.28pm. Outside the domed hall, a dozen cars were on fire, at least one of them containing four suicide bombers.

But intelligence experts believe the third location that the terrorists chose on Monday night in this northeastern suburb of the capital was their main target. Known locally as the Vinnell compound, it is home to scores of former US servicemen who train the Saudi Arabian National Guard. It was the first to be attacked just before 11.20pm, when gunmen ambushed sentries.

When Colin Powell, the US Secretary of State, visited the ruins of the complex yesterday, he walked slowly past an overturned lorry used by the suicide bombers.

At least seven Americans were killed here. General Powell was told that the death toll could be much higher.

About 70 American military specialists employed by the Virginia-based firm of Vinnell live here, some with their families. The company has worked in the kingdom for more than 25 years on contracts worth $800 million (£500 million). A car bomb in December 1995 destroyed a US Army building where Vinnell staff were working.

At their compound yesterday the Stars and Stripes was still flying from the roof of a four-storey block of flats, although all the brickwork had been removed by the blast.

A US Army general described how a lorry and a black car had driven up to the main gates just before 11.20pm, approaching the US-trained men on duty at the front gates.

Gunmen in the car opened fire with automatic weapons, killing four Saudi sentries. One of the terrorists managed to force his way inside the main guardhouse and open the heavy iron gates, allowing the lorry and its explosive load to pass through the security cordon. The lorry was driven a further 250 yards until it reached the highest building in the compound. There it exploded in a sheet of flame.

Some witnesses say that they saw some of the gunmen escaping, including at least one man who had been in the lorry.

Many of the survivors were in their nightclothes and had to run between burning cars, chunks of molten metal and uprooted palm trees to escape. Some had to step over mutilated and burning bodies.

Burst water pipes turned the ground to mud within seconds as ambulances tried to force their way past the jagged rubble to reach the injured. Shielding his eyes against a sandstorm, General Powell stared up at the gutted block of flats yesterday and said: “These are people who were determined to penetrate places like this just for the purpose of killing people in their sleep, killing innocent people, killing people who had tried to help others.”

Families at the al-Hamra village had been given a warning a fortnight ago that Islamic militants were plotting to attack Westerners. Nigel Marks, a computer analyst who lives at al-Hamra, said: “We were told not to go to the shopping complexes in the centre of town, but to stay in the village, where we would be safe. What happened to the promised security?” To make expatriate workers as comfortable as possible at al-Hamra, there are restaurants, bowling alleys, half a dozen swimming pools, a health club, a sports centre and even a race track. The village has its own security force, although robbery in the compound is unheard of.

Al-Hamra was built behind 20ft-high concrete walls with surveillance cameras mounted every few yards. The homes inside vary from eight- bedroom executive villas with their own tennis courts to flats for single men.

Mr Marks was among those allowed back in the ruins of his flat yesterday to collect what personal belongings he could. “When you see what happened here, it is a miracle more were not killed,” he said.

Ten hospitals in the capital were treating the injured yesterday, including Erika Warrington, 15, whose face was bandaged, and Graham Bull, 58, a teacher from Manchester who had injuries to his arm.

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