The immediate question, as ever, is how to prevent such attacks before they happen. The larger question is why, after Afghanistan and Iraq and everything else that has been said and done by western leaders since 9/11, this threat apparently remains so omnipresent - and so scary.
The past few days alone have brought fresh warnings whose non-specific nature only intensifies the vague, nagging sense of menace. In Washington, the Department of Homeland Security raised the spectre of renewed suicide hijackings. Another 9/11-style attack "could be executed by the end of the summer 2003", it said. "Attack venues may include the United Kingdom ... or the east coast of the United States."
US opinion polls indicating falling confidence in Bush's conduct of the "war on terror" found an echo at the UN. Heraldo Munoz, chairman of the al-Qaida sanctions committee, said international collaboration was slipping.
Only 30% of UN members were meeting their obligation to report al-Qaida movements and financing, he said. "Individuals or entities associated with al-Qaida" were still able to acquire weapons and explosives where and when they needed them, as shown by several recent attacks.
Inocencio Arias, chief of the UN's counter-terrorism committee, was hardly more encouraging in a week when a Congressional inquiry criticised 9/11 intelligence failures. "After two years, a lot of people are sleeping again," Arias said. Withholding assistance would be a less tactful way of putting it.
In London, meanwhile, the Commons foreign affairs committee warned that Osama bin Laden still has the capability "to lead and guide the organisation towards further atrocities". The committee also finally reached a conclusion that opponents of the Iraq invasion arrived at long ago: that "the war in Iraq might in fact have impeded the war against al-Qaida", in part by attracting recruits. In any event, threat levels had not been significantly reduced.
The reasons why al-Qaida and like-minded groups have survived the post-9/11 onslaught are discussed by Harvard's Jessica Stern in the latest issue of Foreign Affairs. Al-Qaida has shown a surprising ability to adapt, she argues, by forging new local and regional alliances, embracing additional objectives, changing tactics and eschewing formal hierarchies to encourage "leaderless resistance".
If they are ever to defang and defuse the "totalitarian Islamist revivalism" that constitutes al-Qaida's main inspiration and appeal, Stern says, the US and its allies must exhibit similar adaptability and innovation and more imaginative remedies for east-west alienation.
It is at this point that the doubts about Bush's divisive and frequently crude leadership of the "war on terror" come more sharply into focus. Bush is accused of many things - but never of being imaginative. From the very start, and despite much spin and waffle about fighting a new kind of conflict by unconventional means, Bush has opted for the obvious.
In Afghanistan, nebulous al-Qaida networks posed a complex and subtle challenge. Bush's solution? Invade the country and overthrow its rulers. The Taliban may have had it coming; but that is hardly the point. This was the old-style "overwhelming force" approach long favoured by US presidents, Daddy Bush included.
The Iraq campaign was conducted, for whatever reason (and many were given), on much the same principle: kick the door down, then charge in - and to hell with the wider consequences. While such behaviour brings quick, short-term results and may be superficially gratifying, innovative or imaginative it definitely is not.
These tactics bear little relation to an effective defence against terrorism in the round, let alone to tackling its root causes. Many al-Qaida in Afghanistan were merely dispersed; now they are returning. As for Iraq, they were never there in the first place.
Deputy Pentagon chief Paul Wolfowitz still insists that "Iraq is the central battle in the war on terror". In reality, he is now trying disingenuously to redefine all Iraqi opponents of US occupation as "terrorists" - as somehow one and the same as the people who blew up Manhattan. It won't wash.
The continuing cost of Iraq in terms of ruptured alliances, global tension, economic disruption, Muslim animosity and the daily grief of both occupiers and occupied surely gives great comfort to America's true ideological and cultural enemies. How they must gloat.
The UN secretary-general, Kofi Annan, tried in his usual calming way to put some pieces back together last week. He called for a new sense of common endeavour among nations repelled by Bush's policies in order to meet the challenges posed by global terrorism. Even as he spoke, Bush, discussing Palestine with Israeli prime minister Ariel Sharon by his side, was busy leading his terror crusade down its next blind alley.
Sharon has long sought to portray Israel's conflict with the Palestinians as part and parcel of the US-led "war on terror". Judging by his latest comments, Bush has entirely embraced this view. Terrorism was the issue that overrode all others, he suggested; and peace was conditional on the prior dismantling of all terrorist groups - Sharon's position exactly.
The fundamental flaw in this approach is that, unlike Bush and Sharon, most of the world does not regard Palestinian resistance to occupation in the same light as the activities of al-Qaida and allied transnational groups with their much broader, insurrectionary aims.
Palestinian grievances are specific, easily understood and well-rehearsed. The likes of Hamas and Islamic Jihad are utterly wrong to attack Israeli civilians. But by lumping together Palestinian hardliners with far more virulent international terrorist gangs, Bush confuses the two issues to the detriment of solutions to both.
This blurring of distinctions actually fans extremism and polarisation and the sort of foreign meddling in Palestine by Iran, Syria and Lebanon's Hizbullah that the US so regularly decries. Crucially, by such simplistic analysis, Bush further discredits and undermines international support for his wider anti-terror campaign.
Here once again is Bush's unimaginative "for us or against us" approach, the "good guys v bad guys" routine. Once again he fails to see how daft - and how dangerous - this is. Little wonder that US senators worry that the administration has taken its eye off the ball. With the bumbling Bush as "war on terror" team captain, little wonder if the dread Osama believes he is again staring at an open goal.