Members of the Berlin city fire department attend a ceremony Saturday near the Breitscheidplatz memorial to commemorate fellow firefighters and other victims killed in the 9/11 terror attacks in New York City. Omer Messinger/Getty Images hide caption
People walk near the sight of Ground Zero and the One World Trade Center on Aug. 30. The Wall Street neighborhood changed drastically after the 9/11 attacks as banks moved out of what had long been their home. Spencer Platt/Getty Images hide caption
President Joe Biden speaks about his strategy to combat the coronavirus pandemic at Brookland Middle School on Friday in Washington, D.C. Later in the day, he issued a video commemorating the 20th anniversary of the Sept. 11 terrorist attacks. Manuel Balce Ceneta/AP hide caption
Two men identified by authorities as hijackers Mohamed Atta (right) and Abdulaziz Alomari (center) pass through airport security on Sept. 11, 2001, at Portland International Jetport in Maine in an image from airport surveillance tape released on Sept. 19, 2001. Reuters/Portland Police Department hide caption
The New York City Fire Department's James Gordon Bennett Medal was established in 1869 and features an image of Neptune (left) wading ashore. It will be renamed the Chief of Department Peter J. Ganci, Jr. Medal. New York Daily News Archive/NY Daily News via Getty Images hide caption
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
Members of the Taliban militia ride in vehicles during Afghanistan's annual Independence Day parade in Kabul on Aug. 19, 2001. Afghanistan was largely cut off from the world during the Taliban's rule from 1996 to 2001. That changed dramatically after the Sept. 11 attacks. Saeed Khan/AFP/Getty Images hide caption
The dark stone path that bisects the Flight 93 National Memorial visitors center in Shanksville, Pa., memorializes the flight path of the United Airlines plane. Gene J. Puskar/AP hide caption
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
"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
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 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
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
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
U.S. soldiers pray during the an anniversary ceremony of the terrorist attacks on September 11, 2001 at Bagram Air Field, Afghanistan. John Moore/Getty Images hide caption