The nation pauses Thursday to remember the lives lost 13 years ago in the September 11 attacks.
Commemorations are being held at the site of the World Trade Center in lower Manhattan, at the Pentagon outside Washington, D.C., and in Shanksville, Pa. President Barack Obama is scheduled to speak at an invitation-only event at the Pentagon.
Other ceremonies will be held in communities across the United States.
In an address to the nation a day ahead of Thursday's 9/11 anniversary, Obama said, "We cannot erase every trace of evil from the world, and small groups of killers have the capacity to do great harm. That was the case before 9/11, and that remains true today.''
Photo Gallery: Nation marks 9/11 anniversary