Barack Obama is observing his first September 11 anniversary as president - after inheriting two wars that followed the 2001 terror attacks.
The President was a 40-year-old Illinois state senator on September 11, 2001, and, like his countrymen, he was jarred by what he described as "nightmare images" of destruction and grief that filled the TV that day.
He will visit the Pentagon memorial to those who died there in the 2001 attacks, and meet with loved ones of the dead.
He issued a proclamation on Thursday honouring those who died and urging Americans to mark the anniversary with acts of community service.
He also pledged to "apprehend all those who perpetrated these heinous crimes, seek justice for those who were killed, and defend against all threats to our national security".
The president's challenge, says former Bush foreign policy adviser Juan Zarate, is to "find a balance where he's clearly marking 9/11 as a key historic moment from which his current policies flow, but also not allowing it to define him," as the attacks defined Bush's presidency.
Meanwhile, about 1,000 US troops serving in Afghanistan have run 9.11 kilometres (about 5.5 miles) at the main US base, both to commemorate the anniversary and remember troops who have died in nearly eight years of fighting.
The US and its allies first invaded Afghanistan in October 2001 to oust the Taliban regime for sheltering al Qaida leaders who planned the attacks.
Organisers of Friday's race, which also was held at two other bases, called it an act of defiance against insurgents who have killed more American troops this year than in any other since the beginning of the conflict.
