(CNN) -- A bell tolled, ground zero fell silent.
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 held in New York at 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, where United Flight 93 crashed into a field near Shanksville, a bell tolled after the name of each of the passengers and crew members was read.
Members of the crew of the USS Somerset rang the bells. The Navy named the ship in honor of Flight 93 passengers who fought back against their hijackers. Forty passengers and crew died when the plane went down.
On the steps of the capitol, members of Congress also gathered to mark the occasion. It was the same place where lawmakers gathered 12 years before in the aftermath of the attacks to demonstrate unity.
"This moment is to pray for the families of the departed, and to ask God to renew our strength and replenish our grace, so that we may press on and serve without growing weary, and walk without growing faint towards that more perfect union of our founders dreams," said House Speaker John Boehner. "That is why we return to these steps today, that is why we will always return. And that is why we will never forget."