Look back at some of the work that went into building the memorial to United Flight 93 in Pennsylvania.
Tokens honoring Flight 93 sit near the fence surrounding the permanent memorial's construction site in May 2011.
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Courtesy USA Swimming
Ethan Miller/Getty Images
American Kennel Club via CNN
Ben A. Pruchnie/Getty Images for Walt Disney Studios
Library of Congress [Public domain], via Wikimedia Commons
Frazer Harrison/Getty Images
Darren McCollester/Getty Images
Giuseppe Bellini/Getty Images
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