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.
Library of Congress
REUTERS/San Lorenzo soccer club/Handout
Mike Coppola/Getty Images
Port St. Lucie Police Department
Chip Somodevill/Getty Images
David S. Holloway/CNN
Sign up for Breaking News, Daily Headlines, Severe Weather Alerts & more!