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.
Enrique Castro-Mendivil / Reuters
U.S. National Archives
REUTERS/ Stefano Rellandini
U.S. Air Force
iStock / damircudic
Warner Bros. Television
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