Kevin Lamarque / Reuters

"Certainly the focus will be on remembering that horrific day and remembering the lives that were lost," she said, "and certainly honoring the individuals who were not only lost that day, but also put their lives on the line to help in that process."

Addressing the crowd in a more hushed voice than some of his usual speeches, the president praised the plane's passengers and crew for their heroism.

"A piece of America’s heart is buried on these grounds. But in its place has grown a new resolve to live our lives with the same grace and courage as the heroes of Flight 93," he said.

He said the field where the plane crashed was a "monument to American defiance."

The president also told the relatives and friends of the victims that the country will never forget their loved-ones' heroism.

"America’s future is not written by our enemies. America’s future is written by our heroes," he said.