An artist's image of the wall that will separate the public from the repository where unidentified remains will be kept at the National September 11 Memorial Museum in Manhattan. 911memorial.org hide caption

itoggle caption 911memorial.org

The Willis Tower (left) in Chicago, photographed on Feb. 15. One World Trade Center (right) in New York City, photographed in November. Saul Loeb/Getty Images / Mark Lennihan/AP hide caption

itoggle caption Saul Loeb/Getty Images / Mark Lennihan/AP