Joe Dittmar holds remnants of the World Trade Center — a section from one of the core beams of the South Tower, right, and a bolt from a steel beam. Jud Esty-Kendall/StoryCorps hide caption
Ground Zero Ground Zero
Firefighters work beneath the vertical struts of the World Trade Center's twin towers, in Lower Manhattan, following the attacks of Sept. 11, 2001. Mark Lennihan/Associated Press hide caption
Sept. 11 First Responder Fights On Behalf Of Others Who Rushed To Help
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 hide caption
Sept. 11 Families Face 'Strange, Empty Void' Without Victims' Remains
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 hide caption
For Those Who 'Worked The Pile' At Ground Zero, Horrors Of Sept. 11 Haven't Faded
The 9/11 Responders Remembered Memorial in Nesconset, N.Y. John Feal spends 10 hours a day vetting responders for inclusion on the memorial. Courtesy of FealGood Foundation hide caption
Honoring The Other Fallen Of Sept. 11: Sickened Ground Zero Volunteers
Then: A composite image of Donald Trump (left, clockwise), New York City Mayor Rudy Giuliani, Sen. Hillary Clinton, Tim Kaine, Rep. Nancy Pelosi and Rep. Tim Kaine. Getty Images and AP hide caption
People look out at the former site of the World Trade Center in New York City in 2005, where construction had started on Freedom Tower. Spencer Platt/Getty Images hide caption
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 hide caption
In Ongoing Rebuilding Of Ground Zero, A Balance Of Remembrance, Resilience
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 hide caption
Feliciano stands at the gravesite of his wife, Rosa. "She's buried here," he says. "So Green-Wood Cemetery is ground zero for me." Courtesy of Isaac Feliciano hide caption
At A Brooklyn Cemetery, A Place Of Work — And An Enduring Memorial
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 hide caption
A piece of commercial landing gear was found between two buildings in lower Manhattan. NYPD hide caption
Paul Simon singing 'The Sound of Silence'
President Obama pauses after laying a wreath at the National Sept. 11 Memorial at Ground Zero in New York, Thursday, May 5, 2011. Charles Dharapak/AP hide caption