Americans are remembering 9/11 with moments of silence, readings of victims' names, volunteer work and other tributes 21 years after the deadliest terror attack on US soil.
Victims' relatives and dignitaries will convene on Sunday at the places where hijacked jets crashed on September 11, 2001 - the World Trade Center in New York, the Pentagon and a field in Pennsylvania.
Other communities around the country are marking the day with candlelight vigils, interfaith services and other commemorations. Some Americans are joining in volunteer projects on a day that is federally recognised as both Patriot Day and a National Day of Service and Remembrance.
The observances follow a fraught milestone anniversary last year. It came weeks after the chaotic and humbling end of the Afghanistan war that the US launched in response to the attacks.
But if this September11 may be less of an inflection point, it remains a point for reflection on the attack that killed nearly 3000 people, spurred a US "war on terror" worldwide and reconfigured national security policy.
It also stirred - for a time - a sense of national pride and unity for many, while subjecting Muslim Americans to years of suspicion and bigotry and engendering debate over the balance between safety and civil liberties. In ways both subtle and plain, the aftermath of 9/11 ripples through American politics and public life to this day.
And the attacks have cast a long shadow into the personal lives of thousands of people who survived, responded or lost loved ones, friends and colleagues.
On Sunday, President Joe Biden plans to speak and lay a wreath at the Pentagon, while first lady Jill Biden is scheduled to speak in Shanksville, Pennsylvania, where one of the hijacked planes went down after passengers and crew members tried to storm the cockpit as the hijackers headed for Washington. Al-Qaeda conspirators had seized control of the jets to use them as passenger-filled missiles.
Vice President Kamala Harris and husband Doug Emhoff are due at the National September 11 Memorial in New York, but by tradition, no political figures speak at the ground zero ceremony. It centres instead on victims' relatives reading aloud the names of the dead.