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.
Screen Gems Television
Library of Congress
Patrick Aventurier/Getty Images
Maddie Meyer/Getty Images
Sebastião Salgado via Wikimedia Commons
U.S. Marine Corps
CDC Public Health Image Library
Theo Wargo/Getty Images for Tony Awards Productions
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