Russell Mercer replaces old U.S. flags with new ones at the Flushing World Trade Center Memorial at Flushing Cemetery in New York City. His stepson, Scott Kopytko, was killed on Sept. 11.
Alex Welsh for NPR
A construction worker looks up at One World Trade Center in New York City, the central skyscraper under construction at Ground Zero, a year before its 2013 completion.
Spencer Platt/Getty Images
Three World Trade Center is under construction near One World Trade Center, which was completed in 2013. The new building stands 1,079 feet tall, and its topping-out ceremony was held in June.
Hansi Lo Wang/NPR
Bretagne, shown here at age 13, worked as a search dog at ground zero after the Sept. 11 attacks. She died Monday at 16, the last known surviving Sept. 11 search and rescue canine.
Charlotte Dumas/Barcroft Media via Getty Images
Family members of victims of the Sept. 11 attacks demonstrate against the decision to transfer the unidentified remains to a repository at the World Trade Center site.
Eric Thayer /Reuters /Landov