More than 1000 people gathered on a hot and hazy morning at the National September 11 Memorial plaza in Manhattan, for the annual reading of victims' names from both the 1993 and 2001 attacks on the World Trade Center.
Bagpipes and a youth choir ushered in the start of the solemn proceedings, held around two reflecting pools that stand in the footprint of the fallen twin towers.
"To my nephew Michael Joseph Mullin, we miss you and think of you every single day," said one of the 250 people chosen to read names. "You're gone but not forgotten," another woman said of her lost cousin.
In keeping with a tradition that began last year, no public officials spoke at the New York ceremony, though former New York Mayor Rudolph Giuliani, his successor Michael Bloomberg, Police Commissioner Raymond Kelly and other city and state leaders were in attendance.
In a memorial service at the Pentagon, President Barack Obama called on Americans to pray for those whose lives had been lost. He said: "Let us have the strength to face the threats that endure, different though they may be from 12 years ago, so that as long as there are those who would strike our citizens, we will stand vigilant and defend our nation."
Americans observed moments of silence at 8.46am, the time American Airlines Flight 11 crashed into the North Tower and with a second pause at 9.03am, when United Airlines Flight 175 struck the South Tower.
Further moments of silence were observed at 9.37am, when American Airlines Flight 77 hit the Pentagon; at 9.59am, when the South Tower fell; at 10.03am, when United Flight 93 hit the ground near Shanksville, Pennsylvania; and at 10.28am, 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 US 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 Centre, the tallest building in the Western Hemisphere at 1776ft, a symbolic number chosen to allude to the year of the Declaration of Independence.
At the Flight 93 National Memorial in Pennsylvania, the National Park Service's memorial service included a reading of the names of the Flight 93 passengers and crew, a ringing of bells and wreath-laying.