Families of the victims of the worst terror attack in US history have gathered to mark their 12th anniversary with a moment of silence and the reading of names.
The September 11, 2001 attacks in New York City and Washington killed almost 3000 people, led to a long war in Afghanistan and created an expansion of government surveillance powers that continues to be debated today.
The twin towers later collapsed.
President Barack Obama also marked a moment of silence at the White House and was attending a ceremony in Washington at the Pentagon, which was struck by another hijacked plane.
The attacks on a clear, beautiful morning as commuters arrived at work shook the country's sense of security and of itself.
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"No matter how many years pass, this time comes around each year - and it's always the same," said Karen Hinson, who lost her 34-year-old brother, Michael Wittenstein, in New York. His body was never found.
Obama on Wednesday made no direct mention of the crisis in Syria, but he vowed to "defend our nation" against the threats that endure.
"Let us have the wisdom to know that while force is sometimes necessary, force alone cannot build the world we seek," Obama said during a ceremony at the Pentagon.
The president also paid tribute to the four Americans killed one year ago in an attack on a US compound in Benghazi, Libya, asking the country to pray for those who "serve in dangerous posts" even after more than a decade of war.
In New York, continuing a decision made last year, no politicians were making speeches.
Around the world, thousands of volunteers have pledged to do good deeds, honouring an anniversary that was designated a National Day of Service and Remembrance in 2009.
