As the shock of yesterday's attack sinks in, the recriminations are certain to begin. The principal question for many is certain to be: Why was there no warning?
Just four days ago the US State Department reissued the "worldwide caution" notice that it first put out in June. According to the official statement, there was "an increased risk of a terrorist action from extremist groups". But there was no suggestion that Washington had the slightest idea of what was to befall New York and Washington yesterday.
"Over the last several months, the US government has learned that US citizens and interests abroad may be at increased risk of a terrorist action from extremist groups," the notice said.
"In addition, we have received unconfirmed information that terrorist actions may be taken against US military facilities and/or establishments frequented by US military personnel in Korea and Japan," it continued.
"We are also concerned about information we received in May 2001 that American citizens may be the target of a terrorist threat from Osama Bin Laden's Al-Qaida organisation. In the past, such individuals have not distinguished between official and civilian targets. As always we take this information seriously."
But not, however, seriously enough to do more than issue a relatively pro forma, largely unreported warning of the kind that has become almost routine in recent years. In response to the threat, the US embassies in Tokyo and Seoul alerted local American communities, but neither embassy was closed. With Secretary of State Colin Powell leaving for a two-day trip to Colombia at the start of this week, there is little or no evidence to suggest there was any awareness of the seriousness of what was being planned.
Seven months ago, however, one of America's top counter-intelligence chiefs warned publicly that the US could face the kind of nightmare that erupted.
Admiral Thomas R Wilson told the Senate Intelligence committee that he was "most concerned" about the possibility of "a major terrorist attack against United States interests, either here or abroad, perhaps with a weapon designed to produce mass casualties."
Until yesterday, the US had suffered relatively little from international as opposed to domestic terrorism - the April 1995 Oklahoma city bombing carried out by Timothy McVeigh was by some distance the worst such incident in its history.
The newly reissued worldwide caution notice is in addition to a sequence of State Department public cautions in force covering 20 separate regions or nations around the world in which Americans are considered to be at risk from terrorism.
According to the department's latest review of global terrorism released in April, "terrorism continues to pose a clear and present danger to the international community", but Mr Powell took a relatively upbeat stance on the threat when he gave evidence on the subject to the US Senate in May.
"Americans do not want to hunker down in their embassies and consulates and wait for the next attack, inevitable as that next attack may in fact be," Mr Powell told the committee.
"That outlook is against all that we stand for. It is the very spirit of our people.
"Terrorism is part of the dark side of globalisation. However sadly, it is part of doing business in the world - business we as Americans are not going to stop doing. Indeed, if we adopted a hunkered down attitude, behind our concrete and barbed wire, the terrorists would have achieved a kind of victory."
Since terrorist bombs destroyed two US embassies in East Africa nearly three years ago, the State Department has spent three billion dollars on security initiatives that include shatter-proof windows, high-tech screening devices and plainclothes surveillance teams at embassies around the world. Mr Powell is proposing an additional 1.3 billion dollars for even more security upgrades in the coming fiscal year.