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Flowers and American flags are arranged on the north pool of the National September 11 Memorial during a ceremony marking the 10th anniversary of the attacks at the World Trade Center, Sunday, Sept. 11, 2011 in New York. (AP Photo/Mary Altaffer)
More than 4 million people have visited the Sept. 11 memorial at the World Trade Center since it opened last summer on the 10th anniversary of the terror attacks.
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