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.
Marek Szczepanek/Wikimedia Commons
Larry Busacca/Getty Images
Warner Bros. Television
HRH The Duchess of Cambridge via Getty Images
iStock / MartiSaiz
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Adele.com via CNN
National Nuclear Security Administration
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