President Obama, first lady Michelle Obama, Vice President Joe Biden and others observe a moment of silence on the South Lawn of the White House on Thursday to mark the 13th anniversary of the Sept. 11 attacks. Charles Dharapak/AP hide caption

itoggle caption Charles Dharapak/AP

"Timescape" finds words in the news associated with Sept. 11, and weights them according to prominence in a story — not just how often they appear. Gaurav Bradoo hide caption

itoggle caption Gaurav Bradoo

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

itoggle caption Eric Thayer /Reuters /Landov

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

itoggle caption Gary Hershorn/Reuters/Landov

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

itoggle caption Justin Lane/pool/Getty Images

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

itoggle caption Alex Wong/Getty Images

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

itoggle caption Pool/Getty Images

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

itoggle caption Andrew Burton/Getty Images

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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