A woman looks out at One World Trade Center from inside the 9/11 Empty Sky memorial at Liberty State Park in Jersey City, N.J., on Wednesday. Americans commemorated the 12th anniversary of the Sept. 11 attacks with solemn ceremonies and pledges to not forget the nearly 3,000 people killed. Gary Hershorn/Reuters/Landov hide caption

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Robert Peraza, who lost his son Robert David Peraza, pauses at his son's name at the North Pool of the 9/11 Memorial at the site of the World Trade Center. Justin Lane/pool/Getty Images hide caption

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Singer Patti LaBelle performs during the "Concert For Hope" event, hosted by Washington National Cathedral, to mark the tenth anniversary of the 9/11 terrorist attacks. Alex Wong/Getty Images hide caption

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The Two-Way

Musical Moments: Alan Jackson, Patti Labelle, Denyce Graves

Three performances from the Sept. 11 memorial "Concert of Hope."

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Keri McMorrow, 7, visits the memorial pool where her uncle's name is engraved, during tenth anniversary ceremonies of the September 11, 2001 terrorist attacks at the World Trade Center site. Pool/Getty Images hide caption

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President Obama looked out at the North Pool of the 9/11 Memorial in New York earlier this morning. Andrew Burton/Getty Images hide caption

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The Two-Way

A Musical Moment Of Reflection: Cellist Yo-Yo Ma At Ground Zero

LISTEN: He performed Sarabande from Bach's First Suite for Cello Solo.

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The "Tribute in Light" illuminates the sky in New York on the 10th anniversary of the Sept. 11 attacks. Timothy A. Clary/AFP/Getty Images hide caption

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Rick King, who was assistant fire chief of Shanksville, Pa., on Sept. 11, 2001, stands near a cross made from steel from the World Trade Center, outside the fire station in Shanksville on July 14. He was one of nine people to tell NPR what Sept. 10 was like, the day before the horrible events of Sept. 11. Gene J. Puskar/AP hide caption

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Outside the Pentagon, 184 benches are part of the memorial honoring those killed there on Sept. 11, 2001. Jose Luis Magana/AP hide caption

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