Americans marked the 12th anniversary of the September 11 attacks with solemn ceremonies and pledges not to forget the nearly 3,000 killed when hijacked jetliners crashed into the World Trade Center, the Pentagon and a Pennsylvania field, in 2001.

In New York City, still scarred from the attacks that brought down the Twin Towers, residents repeated the ritual on Wednesday of reading the names of the 2,983 people who died that day.

Officials, including former Mayor Rudolph Giuliani and his successor Michael Bloomberg, observed a citywide moment of silence at 8:46 a.m. EDT, the time American Airlines Flight 11 crashed into the North Tower, with a second pause at 9:03 a.m. when United Airlines Flight 175 struck the South Tower.

The ceremony was held at the National September 11 Memorial plaza, where two reflecting pools mark the footprints of the original Twin Towers.

Further moments of silence will be observed at 9:37 a.m., when American Airlines Flight 77 hit the Pentagon; at 9:59 a.m. when the South Tower fell; at 10:03 a.m. when United Flight 93 hit the ground near Shanksville, Pennsylvania; and at 10:28 a.m., when the North Tower collapsed.

Nineteen hijackers died in the attacks, later claimed by Osama bin Laden and al Qaeda, which led directly to the U.S. war in Afghanistan and indirectly to the invasion of Iraq.

Two skyscrapers are nearly completed on either side of the plaza, including One World Trade Center, the tallest building in the Western Hemisphere at 1,776 feet, a symbolic number chosen to allude to the year of the Declaration of Independence.

In Washington, President Barack Obama and Vice President Joe Biden marked the first moment of silence on the South Lawn of the White House, as a bell tolled.

"Friends of mine who lost friends in a different context long ago have a simple saying: 'Every day is extra,'" said Secretary of State John Kerry, who served in the Vietnam War. "I've always thought it was a beautiful expression and I try to hold on to it on bright September mornings like today. It's a way of saying that we honor those we've lost by continuing their work, serving our country and helping others."

At the Flight 93 National Memorial in Pennsylvania, the National Park Service planned a memorial service on Wednesday starting at 9:45 a.m. that will include a reading of the names of the Flight 93 passengers and crew, a ringing of bells, a wreath-laying and brief remarks.

A number of cities will hold ceremonies, including 21-gun salutes, moments of silence, and memorial services Wednesday to remember those who lost their lives, including the police officers and firefighters killed as they attempted to save the victims of the attacks.

The website 911memorial.org is encouraging classroom discussions, service projects, and other commemorative acts.