A heavily armed Port Authority police officer stand guard next to the North Pool at the World Trade Center memorial site Friday, Sept. 9, 2011 in New York. Just days before the 10th anniversary of the Sept. 11 attacks, U.S. counterterrorism officials are chasing a credible but unconfirmed al-Qaida threat to use a car bomb on bridges or tunnels in New York City or Washington. (AP Photo/Mary Altaffer)
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