Names of suspected terrorists with possible ties to Osama bin Laden were found on the passenger lists of the planes used in the shattering assault on the United States' key symbols of military and financial power.
Bin Laden, the Saudi exile believed responsible for the 1998 bombings of two US embassies in Africa, had warned three weeks ago of an "unprecedented attack" on US interests.
But US officials said they had no advance warning of the attacks, which plunged the country into chaos and panic, and reverberated through international air travel and financial markets.
The US virtually closed down after the worst attack on American soil since Japan's raid on Pearl Harbor that brought the US into the Second World War.
US forces in Europe and the Middle East went on the highest state of alert as Governments worldwide held crisis meetings, tightening security at embassies, airports and public buildings and braced for more possible attacks.
Planes bound for the US were recalled in mid-flight, Britain stopped civilian flights over London, and Nasa closed the Kennedy Space Center. Foreign embassies in Washington closed or sent US personnel home.
Egypt, Israel and European leaders from Britain to Russia broke off normal business for crisis meetings.
Officials throughout the day said the massive attacks by hijackers who crashed commercial airplanes into the World Trade Center and the Pentagon may be linked to Osama bin Laden and his al Qaeda organisation.
Senator Richard Shelby, an Alabama Republican who is vice-chairman of the Senate Intelligence Committee, said: "A lot of things point to him." "There is reason to believe people with links to him may have been responsible for this."
But some intelligence analysts urged caution in making snap judgments about who was responsible, saying it could have been the work of other groups tied to the Middle East, angry over a perception that the US supports Israel to the detriment of the Palestinians.
Afghanistan's Taleban rulers quickly denied that bin Laden, whom they are sheltering, was capable of launching the attacks from inside Afghanistan. Taleban supreme leader Mullah Mohammad Omar condemned the attacks as terrorist acts.
Libyan leader Muammar Gaddafi, regarded as a pariah by Washington, offered condolences at the "horrific attacks". Iranian President Mohammad Khatami offered "deep sympathy". Palestinian President Yasser Arafat condemned the attacks as a "terrible act".
Saddam Hussein's Iraq raised a dissenting voice, saying the US deserved the attacks. "The American cowboy is reaping the fruits of his crimes against humanity," Iraqi state television said.
Russian President Vladimir Putin and British Prime Minister Tony Blair called for a coordinated international response to terrorism. Chinese President Jiang Zemin sent a message to Bush expressing sympathy.
In Europe, France called a general security alert and Germany convened its national security council, cancelling debate in Parliament for the rest of the week.
Japanese intelligence officials were investigating whether there was any link between the attacks and unconfirmed information that the US embassy in Tokyo had received last week of a possible terrorist attack.
Australia's Government spent much of the day working on ways to get Prime Minister John Howard home from the US. Acting Prime Minister John Anderson said defence aircraft might be deployed to ensure Howard's return.
The vicious attacks provided deadly proof of the power and sophistication of the world's most determined terrorists. They also heightened doubts about the planned missile defence shield to guarantee security against rogue states. The project focuses on tracking and destroying inter-ballistic missiles before they reach their targets.
The events of yesterday prove that the strategy is ultimately fruitless, as well as being extremely costly both in financial and in political terms.
Even if it can be made to work, it gives no protection against a more likely form of attack: international terrorism.
- INDEPENDENT, AGENCIES
Full coverage: Terror in America
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The fatal flights
Emergency telephone numbers for friends and family of victims
These numbers are valid for calls from within New Zealand, but may be overloaded at the moment.
United Airlines: 0168 1800 932 8555
American Airlines: 0168 1800 245 0999
NZ Ministry of Foreign Affairs and Trade: 0800 872 111
US Embassy in Wellington (recorded info): 04 472 2068
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Air New Zealand flights affected
Vicious strike heard around globe
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