Americans across the country paused Wednesday to remember the victims killed 12 years ago in the worst terrorist attack in U.S. history.
In New York, relatives recited the names of the nearly 3,000 people who died when hijacked jets crashed into the World Trade Center, the Pentagon, and near Shanksville, Pa.
President Barack Obama and Vice President Joe Biden marked the anniversary with a moment of silence on the South Lawn. They walked out of the White House at the exact moment the first plane hit the World Trade Center tower.
The commemorations will end with the Tribute in Light display, beginning at nightfall from New York's Ground Zero.
On 911day.org, thousands have already pledged to do some type of benevolence work this day to honor the victims.
Meanwhile, New York Police Commissioner Ray Kelly said the threat of terror in the Big Apple still exists.
"Terrorism is as great, if not greater, than before the World Trade Center was destroyed," Kelly said.