It was the attack Britain's police and intelligence chiefs had been predicting, and dreading. Despite spending millions of pounds on counter-terrorism, having some of the world's most advanced surveillance equipment, and a nation supposedly on high alert, the terrorists succeeded yesterday in getting through the UK's defences.
Up until now, the police and security services have been congratulated at foiling any terrorism act since the 11 September 2001 atrocities. Police say they have thwarted five major al-Qa'ida plots in the UK. But they are still struggling to get anything like a full understanding of the al-Qa'ida threat in the United Kingdom.
Lord Stevens, the previous commissioner of the Metropolitan Police, was not thanked for saying last year that an attack by Islamist extremists in London was "inevitable". But yesterday's bombings showed just how difficult it is to stop a determined opponent, especially with the gaps in intelligence.
Knowledge about the overall strength and structure of al-Qa'ida and its associates in Britain is, at best, patchy and blurred. Scotland Yard has said about 200 British citizens and foreign nationals have travelled to al-Qa'ida training camps and returned to the UK. But intelligence officers believe the true figure could be up to 3,000.
Most of these Islamic extremists are known to the authorities, but a significant number have almost certainly returned without being traced. This was illustrated by Idris Bazis, a French-Algerian living in the Moss Side district of Manchester. The 41-year-old came to the attention of the British authorities only after he travelled to Iraq and committed a suicide bombing attack in February this year.
One of the most alarming developments for the intelligence agencies has been the radicalisation of young British-born Muslims. MI5 and the anti-terrorist branch have become increasingly concerned at the number of these would-be terrorists who have no history of violence or extremism.
In March this year Saajid Badat, 25, admitted plotting to blow up an aircraft bound for America in the first major prosecution of an al-Qa'ida terrorist in the UK since the 11 September attacks. The British-born Muslim with no history of extremism had planned to detonate a bomb at the same time as the London-based "shoe bomber" Richard Reid, who is serving a life sentence in the United States. But Badat changed his mind and dismantled his shoe bomb, which was seized by police at his home in Gloucester in November 2003.
Anti-terrorist officers believe Badat became radicalised by extremists at a mosque in south London in the late 1990s after he fell out with his father. He then travelled to Afghanistan and spent two years at an al-Qa'ida training camp. In a separate case, which has yet to come to court, several young Muslim men with no history of extremism have been charged with plotting to make a bomb in the south-east of England.
Another problem for the UK's counter-terrorism officers is the random structures of al-Qa'ida groups and supporters in Britain. Some cells are closely aligned to Osama bin Laden, but others are merely sympathetic towards al-Qa'ida's aims. There are also loose-knit affiliations and lone fanatics.
The tactics adopted by the extremists are varied and therefore difficult to guard against. Police have long been fearful of an attack by suicide bombers.
New guidelines were drawn up by Scotland Yard in 2003 by the Met's Deputy Assistant Commissioner Barbara Wilding, who travelled to Israel and Sri Lanka to gather first-hand accounts from police and intelligence agencies on how to hunt down and guard against suicide bomber cells. Her confidential report in which she effectively calls for a "shoot-to-kill" policy - has been circulated to chief constables. One of their biggest operations resulted in an al-Qa'ida supporter being jailed for 17 years in April this year for leading a plot to terrorise Britain with ricin and cyanide. As he was sentenced at the Old Bailey, the court heard that Kamel Bourgass, an Islamic extremist from Algeria, had been convicted last year of murdering a Special Branch officer, Stephen Oake.
Bourgass, who is serving a life sentence for the murder of DC Oake during an anti-terrorist operation in Manchester two years ago, was convicted of plotting to produce ricin from a flat in Wood Green, north London.
Bourgass was recruited, inspired and guided by Abu Doha. Doha, 39, who was living in London, was a member of the Salafist Group for Call and Combat (GSPC), a terrorist group which has committed widespread atrocities in Algeria. In 1998, a US indictment says, he won permission from Osama bin Laden to set up the Khalden training camp in Afghanistan for Algerians and other north Africans. Doha was arrested at Heathrow in February 2001 trying to board a flight to Saudi Arabia with a false passport. He is in Belmarsh prison, south-east London, fighting extradition to the US.
Since the 11 September attacks, much of the police and security services' work has been aimed at disrupting and deterring suspected al-Qa'ida followers and supporters through raids and arrests. These operations have often been aimed at breaking up suspected cells, fund-raisers and sympathisers who provide logistical support such as safe house and forged papers.
Intelligence officers admit privately that sometimes they have had to act prematurely because of the risk of a terrorist act. This has resulted in some suspects being released without charge or facing relatively trivial offences. A significant number of innocent people have also been caught up, which has led to considerable bad feeling among Muslim communities who increasingly feel victimised.
Of the 702 people arrested under the Terrorism Act 2000 since the 11 September attacks and the end of last year, comparatively few convictions of terrorists have resulted. Of those arrested, 119 were charged with terror offences and a further 135 with other crimes. Only 17 have been convicted so far under the Terrorism Act.
Arrested in Britain
Jailed in April for 13 years. Badat planned to set off a shoe bomb on an aircraft bound for America but changed his mind. The device was found in a raid on his Gloucester home.
The British Muslim is serving a life sentence in the US for his December 2001 attempt to blow up a flight from Paris to Miami.
An Algerian who stabbed to death Detective Constable Stephen Oake and planned poison attacks was jailed for 17 years in April.
Held at Belmarsh prison, southeast London, since February 2001 accused of directing a plot to bomb Los Angeles airport.
London-based cleric released from Belmarsh and placed under house arrest. Linked by Spanish judges to the Madrid bombingsReuse content