Source: Canada and NATO: The Campaign Against Terrorism
Ministry of Foreign Affairs, Canada.
http://www.dfait-maeci.gc.ca/foreign_policy/nato/nato-en.asp
 

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The North Atlantic Council, NATO’s top decision-making body, invoked the spirit of Article 5 following a debate at NATO headquarters on September 12, 2001 ? a clear demonstration of tremendous solidarity within the alliance. The Council decided that if it was determined that the attacks were directed from abroad, they would be actions covered by Article 5 of the Washington Treaty. Subsequently, on October 2, 2001, the U.S. Ambassador at Large and Coordinator for Counterterrorism briefed the North Atlantic Council on the results of investigations into the September 11 terrorist attacks against the U.S. As a result of the information he provided to the Council, the Council decided that the U.S. had presented clear and compelling evidence that the individuals who carried out the attacks belonged to the worldwide terrorist network of Al-Qaeda, headed by Osama bin Laden and protected by the Taleban regime in Afghanistan.

At a special press conference, NATO Secretary-General Lord Robertson announced that since it had been determined that the attacks had been directed from abroad, they were regarded as an action covered by Article 5 of the Washington Treaty.