It was the nightmare everyone in the counterterrorist agencies dreaded: a coordinated series of bomb attacks in the heart of London at rush hour, intended to kill and maim and bring the transport system to a halt.

Only an hour after the Metropolitan Police commissioner, Sir Ian Blair, was quoted on BBC radio as saying that his force’s anti-terrorist preparations were “the envy of the world,” the bombers showed how hollow such a claim can sound. Now Scotland Yard has launched a manhunt in an attempt to track down the terrorists who carried out yesterday’s carefully timed attacks.

Investigators are examining statements on an Islamic Web site run by the previously unknown Secret Organization Group of Al Qaeda of Jihad Organization in Europe, which claimed responsibility for yesterday’s bombings and intimated that attacks in Denmark and Italy could be next, according to the BBC. Scotland Yard’s deputy assistant commissioner, Brian Paddick, said that British intelligence could not determine whether the attacks were carried out by suicide bombers, and stressed that no government agency received any warning regarding yesterday’s attacks, the BBC reported.

Ever since the September 11, 2001, atrocities in America, there have been warnings that an attack on London was “inevitable.” There have been predictions of chemical or biological attacks and exercises have been carried out on the Underground to try to counter them. When it came, the assault bore the bloody signature of Al Qaeda: a series of bombings on soft targets designed to cause maximum carnage, fear, and panic, as in Bali and Madrid.

A former London police commissioner, Sir John Stevens, said last year that it was not a matter of “if but when” London was attacked.

Politicians wanted to avoid making people jumpy, and the longer the capital survived without an attack, the louder the voices of skepticism could be heard. Was the government not overreacting by locking up suspected foreign terrorists without trial? Was President Bush overdoing the talk of the “war on terror”?

Three years ago, when tanks were sent to Heathrow after fears that an attempt was to be made to shoot down an airliner with a surface-to-air missile, the scale of the military response was an embarrassment to the British government and led to questions about its timing just before the war in Iraq.

But the threat was real. MI5 had picked up sufficiently worrying intelligence to justify briefing Prime Minister Blair and senior ministers about the possibility of an imminent attack.

Leading anti-terrorist specialists have long been convinced that Islamic terrorists were planning a “spectacular” attack in Britain. Their prediction: London’s transport system in the rush hour.

Their suspicions proved correct, at a time when London was celebrating the success of its bid for the 2012 Olympic Games, although the terrorists had probably long planned the attack to coincide with the G-8 summit.

It was not the first planned attack on the capital; others have been thwarted. Potential bombs have been intercepted and at least two suspected terrorist groups broken up and a series of arrests made, with trials due to start this year.

Although yesterday’s bombings showed clearly that complete security cannot be guaranteed, patrols on the Undergound and at mainline stations have been noticeably stepped up in recent months.

Transport is not the only target favored by Al Qaeda fanatics. Clubs and bars, as in Bali in 2002, where more than 200 were killed, are also vulnerable.

To the terrorists, the geographical whereabouts of the attack is not necessarily important. Turkey has been a target several times because it is a secular Islamic state – something the fundamentalists loathe – and because the bombers came from Turkey or thereabouts. Al Qaeda uses indigenous Islamists to pursue grievances within the state and to act as proxies for the world wide “struggle” against the West and its values.

The attack in Bali was carried out by Indonesian Islamists, killing mostly Australians and 28 Britons. Bombs in Saudi Arabia were detonated by local groups.

Unlike with the Irish Republican Army, the profile of the Al Qaeda bomber is becoming harder to discern. Over the years, MI5 and the police began to get the measure of the IRA and while sometimes the bombs got through to devastating effect, as in the city of London, Docklands, and in Manchester, a combination of informers and effective intelligence work foiled other plots.

With Al Qaeda, MI5 and the police face greater difficulties. Today’s Islamist terrorist could be a foreigner or homegrown, but he will be of an ethnic and religious background that makes infiltration difficult by white police agents and requires better intelligence from the community in which he lives.

As has now been seen in London and elsewhere, the new-style terrorists never issue warnings; they seek maximum carnage and may often be suicide bombers. Beyond the extension of the “struggle” worldwide, they have no obvious political aims that anyone can begin to address.

For Osama bin Laden’s followers, the favored terrorist methods are relatively straightforward and almost impossible to guard against.

While the bombers may be from overseas, counterterrorist agencies have been worried for some time about a radicalized new generation of British Muslim youth, which has been further politicized by the preaching of fundamentalist imams and by the prominent position taken by Britain in the conflicts in Afghanistan and Iraq.

British-born Muslims have traveled to Israel to act as suicide bombers, and a London convert, Richard Reid, is in jail in America for trying to blow up an airliner.

Homegrown extremists, nursing grievances against the country in which they live, represent a worrying development. Whenever this point is made, however, the political backlash is intense.

Last year, the former archbishop of Canterbury, Lord Carey, provoked a storm of anger when he denounced moderate Muslims for failing unequivocally to condemn the evil of suicide bombers.

An opinion poll suggested that many Muslims regarded the war against terrorism to be anti-Islam and thought that anti-terrorist laws were being used unfairly against the Muslim community.

Most disturbing was the finding that 13% of British Muslims said they believed further terrorist attacks on America were justified.