Key questions on the tactics of these terror attacks

Why did the attacks fail?

Although the two London car bombs were rudimentary, depending on a lethal mixture of petrol, gas canisters and nails, they could still have killed hundreds.

They were intended to be triggered by calls to mobile phones left in the cars. Although the bombers rang the phones several times, the bombs failed to go off. Did the calls fail to create the necessary detonation?

The Glasgow attack appears to have been a failed suicide bombing. The Jeep Cherokee that smashed into the city's airport was set alight but the gas canisters inside failed to ignite.

Why did the tactics differ in the two cities?

The failed London attacks were carefully planned. One theory is that the bomb in Cockspur Street could have gone off after the Haymarket device, hitting people fleeing the first explosion. The Glasgow attack seems more improvised. The bombers could have hastily constructed a car bomb knowing the police net was rapidly closing around them. They could also have decided to try to detonate the bomb manually rather than by remote control following the London failures.

What were the links between the two?

The English and Scottish blasts were co-ordinated and probably timed to coincide with the handover of power from Tony Blair to Gordon Brown. The plotters - all thought to be foreign - appear to have been known to each other and operating as a cell. Police are establishing more and more links between them.

Were the alleged plotters previously known to police?

Detectives were last night questioning eight people in connection with the attempted attacks. Most of them, including the doctors in custody, appear to have been "clean skins" - unknown to police and not fitting the traditional terrorist profile. There are suggestions that one of them had been questioned but released during the investigation into Dhiren Barot, who was jailed this year over a vehicle bomb plot including targets in London and the US.

What warnings had there been of planned attacks?

Although police were taken by surprise by the failed attacks, intelligence chiefs had singled out the moment of the Blair-Brown handover as a prime target. In April, the Joint Terrorism Analysis Centre said it had no specific intelligence about a planned atrocity, but warned about contacts between al-Qa'ida operatives in Iran and British sympathisers. It concluded that an attack could be in the offing, possibly timed to coincide with Mr Blair's departure. Just hours before the London car bombs were left, a message appeared on a Jihadi website, proclaiming: "Today I say: rejoice, by Allah, London shall be bombed."

Were the attacks directed from abroad?

The evidence points in that direction and ministers have explicity blamed "international terrorism". The suspects are all foreign and the failed attacks bear the hallmarks of car bombs and suicide attacks that have become familiar in parts of the Middle East, Iraq and Afghanistan.