Kim Sengupta: The mystery of how the best-laid plans could end in panic and failure

It was an alleged international terrorist sleeper cell, drawn from the world of professional medicine, maintaining perfect security cover. And it came within a successful mobile phone call of carrying out the devastating "spectacular" al-Qa'ida have been threatening.

Experts in security circles say they have never come across anything like it before. Yesterday, as arrests continued, and more suspect cars were blown up, intelligence specialists were expressing their amazement at the sheer audacity and ambition of the alleged plot.

The suspects form a disparate group of people of different nationality. Medicine and the Muslim faith are the only common factors between them. One emerging hypothesis in the security community is that they were sent to the UK to blend into the community, and wait for the right time to attack.

The group were in a profession dedicated to saving, rather than taking, lives; they were sent to a country whose health service is in need of help from abroad; they settled in areas outside London, well away from radical Muslim hotspots.

As a result, the cell remained below the radar of the police and the security service during one of the most intense anti-terrorist offensives in British history.

But why did a plot with so much foresight fail? The answer is likely to be that the very secrecy which protected the cell from scrutiny prevented it from being helped in the planning and bomb making by an outside expert.

There is, say the sources, no credible evidence so far that any "master bomber" was in contact prior to the attacks. This was because of fears, perhaps, that such a person could compromise the safety of the cell. Instead, the members of the group operated with complete autonomy and were allowed to carry out their own mission.

The devices placed in the two Mercedes cars in central London and the Jeep in Glasgow - propane gas cylinders, petrol and nails designed to be detonated by mobile telephones - are likely to have been chosen from the internet. These devices would not have been an experienced terrorist's first weapon of choice.

Much has been made of the "Iraq connection". The two suspected would-be London bombers were Iraqi doctors living in Glasgow, and car bombs have become synonymous with Baghdad. But, crucially, car bombers in Iraq would use explosives with much more immediate and lethal effect.

Investigators also dismissed suggestions that the cell had been instructed from inside prison by Dhiren Barot, who was convicted for plotting car bombings in the so-called "Gas Limos Project" in November last year.

Barot pleaded guilty at his trial. His defence team maintained in mitigation that his plans could and would not have worked. That did not save Barot from a 30-year sentence. But explosives experts agreesuch a plan would be unreliable as the means of a bombing campaign. The gas method involved opening the cylinder tops and letting the vehicles fill with vapours, which would then be ignited by mobile phone calls. That, in turn, would explode the gas cylinders, creating a fireball spraying petrol and nails.

If all that worked, the effect would have been hugely destructive. The bombers, however, made four calls to the mobile in the Mercedes parked outside the Tiger Tiger nightclub and two to the one in Cockspur Street without managing to detonate the gas and petrol device. In fact, the gas vapour in the car outside the Tiger Tiger car became so noticeable that it led to the discovery of the bomb.

Furthermore, the wealth of evidence left in the two intact Mercedes saloons led to the raids and arrests across the country. After meticulously planning for so long, the cell had chosen one of the crudest and riskiest means of conducting a terrorist attack.

Robert Emerson, a security analyst, said: "What was being planned, allegedly, with the use of doctors was reprehensible. But it was also impressive until at the end, when they simply used an unreliable method. But other terrorists who follow will learn from these mistakes."

Questions still to be answered

Why did the cell choose such a crude device?

One theory is that the men may have been operating beyond known Islamist channels to avoid detection. Instead it is possible they gleaned their bomb-making information from the internet. That may explain a number of errors, including the use of mobile phones in the devices with stored numbers that led police straight to the suspects.

Was Dr Mohammed Asha a ringleader?

Muslims from wealthy, academic backgrounds like his have certainly been radicalised after moving to the West and Dr Asha was certainly the most brilliant of the suspects now known to police. He seems to have planned to leave his Staffordshire home imminently.

Was it really possible they were entirely off the radar?

Scotland Yard's Counter-Terrorism Command and the Security Service insist there was no intelligence warning of last week's attacks, despite reports from America that there had been a specific warning of an attack on Glasgow Airport. But it is possible some of the suspected bombers had been identified in surveillance operations only to be considered too low a priority to pursue.

Was there any British involvement?

According to reports, police are looking for several British nationals or naturalised British citizens who were part of the bombing plot.

Cahal Milmo and Ian Herbert