At 8:46 a.m., hundreds who gathered at the site of the fallen World Trade Center towers paused in silence to mark the moment when American Airlines Flight 11 crashed into the north tower -- the opening salvo of a terrorist attack that brought down the iconic buildings, killed 2,977 people and launched more than a decade of war.
Bagpipers broke the silence, and family members of victims of the 1993 bombing of the World Trade Center and the 2001 attack began a solemn reading of the names of those killed at the site.
The 9/11 attack killed 2,753 people in New York, including 403 police and firefighters. The 1993 bombing killed six people.
In Washington, President Barack Obama, Vice President Joe Biden and their wives also paused in silence outside the White House to commemorate the 9/11 victims. The Justice Department also held a moment of silence.
Another moment of silence was planned in New York for 9:03 a.m., when the second jetliner, United Airlines Flight 175, crashed into the World Trade Center's south tower.
And at the Pentagon, where American Airlines Flight 77 crashed at 9:37 a.m. on September 11, Obama laid a wreath and then spoke at a private observance for family members of the 184 people who died there.
"Our hearts still ache for the futures snatched away," he said.
In southwestern Pennsylvania, it is only expected to take 18 minutes to lay a wreath and read the names of the 40 people who died there, beginning at 9:45 a.m. and ending at 10:03 a.m.
The latter is the time United Airlines Flight 93 crashed into a field near Shanksville. The target of those hijackers is unknown, but the hijackers apparently crashed the plane short of the target because they feared losing control of the plane to some of the 40 passengers and crew who were attacking them.