The US President George W Bush said he had taken all "appropriate steps" to protect the American people.
He said the US would hunt down and punish those responsible for the attacks on the World Trade Center and the Pentagon.
"We have taken all appropriate security precautions to protect the American people," Mr Bush said at Barksdale Air Force Base in Louisiana.
"Make no mistake, the United States will hunt down and punish those responsible for these cowardly acts."
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Mr Bush said he had taken security precautions to ensure the functioning of the US government.
"We will do whatever is necessary to protect America and Americans."
"The resolve of our great nation is being tested. We will show the world we can rise to that challenge."