Who were the bombers?

Theory 1

'The home-grown bombers'

The bombers are self-taught British citizens. They may have been radicalised - through contact with a radical ideologue of some sort - or simply self-starting, full of anger at the war on Iraq or other policies. They learnt their bombmaking expertise from the internet or were possibly taught by someone who had trained in Afghanistan or Pakistan during the Nineties. They have few overseas contacts.

Theory 2

'The foreign professionals'

The attackers were a group of experienced operatives from overseas who were already involved in extremist violence, possibly in one of the various 'theatres of jihad' around the world such as Kashmir or Chechnya. They are likely to be illegal immigrants who are plugged into networks such as those existing in the Algerian community in London. They may have been previously based in France or Germany or Spain and have travelled to the UK specifically in order to execute a terrorist attack. However, they would almost certainly have needed local support.

Theory 3

'Iraqi bleedback'

The core of the terrorist cell was composed of British citizens who have returned from fighting or training with insurgents in Iraq. American terrorism analysts say that the bombs in London bear some resemblance to the later models of improvised explosive devices which have wrought mayhem in Sunni areas of Iraq over the past two years. They could have recruited help on their arrival back in the UK from others committed to the radical cause.

Theory 4

'Forget the categories'

Analysts say that, as modern Islamic militancy is a nebulous, varied phenomenon based on social links, associative networks and small group dynamics, any terrorist cell will be made up of different types of people with different backgrounds - though they will all share the same commitment to violence in the name of religion.