Who was behind the bombings? The four key theories

Evidence is growing that small numbers of young Muslims in the UK are willing to adopt violence in protest against events such as the war in Iraq.

Because these people do not have any history of violence or involvement in terrorism, it is hard for intelligence agencies to investigate them.

One group of British-born Muslims is awaiting trial on terrorism charges involving a plot to build a bomb in the UK.

While MI5 has become alarmed at this new category, the skill and equipment needed to make four high-explosive bombs, and to set them off at the same time, makes it unlikely that the group responsible for Thursday's attack acted without foreign help.

THE FOREIGN CELL

The need for knowledge about how to make reliable explosive devices suggests that the bombers either had help from foreign terrorists or were themselves seasoned fighters.

Activists in al-Qa'ida, or sympathetic to Osama bin Laden, could have travelled to Britain months ago from France or Spain. Here they may have stayed in safe houses or received forged papers from al-Qa'ida supporters.

Such teams are hard to track as they are not part of a formal network. Of the eight terrorist plots the police say they have foiled since 11 September 2001, not one involved the same group. The possibility of a sleeper al-Qa'ida hit squad, sent years ago, seems remote.

THE MADRID TEAM

Similarities between the bombings of commuter trains in Madrid and the Tube and bus attacks in London raise the possibility that they are linked. As in Madrid, the bombs appear to have been in rucksacks left during the morning rush-hour.

Several suspects from the Madrid cell are still at large and some are thought to have fled to the UK. Among them is Mustafa Setmariam Nasar, 47, a Syrian who is believed to be the mastermind of the attacks in Spain. He has lived in London and has connections with Britain going back 10 years.

The Spanish security services have been providing intelligence and help to MI5 and the Metropolitan Police, but as yet no link has been established.

BRITISH AL-QA'IDA

A strong possibility is that the bombers were either headed or advised by a British-born Muslim trained at an al-Qa'ida camp in Afghanistan or Pakistan.

Back in the UK, he would have gathered other fanatics radicalised by extremist clerics and passed on the terrorist skills and organisation.

The police think about 200 extremists have travelled abroad and returned to the UK with terrorist skills.

Saajid Badat, 26, a British-born Muslim who was trained at al-Qa'ida camps, admitted plotting in 2003 to blow up an aircraft bound for America. Jailed in April 2005 for 13 years, Badat had planned to set off a shoe bomb but changed his mind and dismantled it.