Friday's milestone anniversary will bring memorial rituals — wreath-laying, flag-flying, moments of silence, vigils and readings of victims' names — and volunteer projects in tribute to a day that about half of US adults remember as the most historic event of their lifetime, according to a new poll.
For others it's an occasion for private reflection — or, in many schools, education — about the attack that unfolded in a single morning but left nearly 3000 dead and a decades-long trail of aftermath, from personal grief to far-flung wars.
Police Officer Casey Kloepfer plans to spend time Friday at the Brooklyn station house where his father, Officer Ronald Kloepfer, reported to work that day before being called to the World Trade Center. He didn't make it home.
Casey Kloepfer was only four, but he grew up hearing about his father's affinity for helping people and for a job that enabled him to do so. He became an officer this past June.
"It's big footsteps to fill, but it's definitely something that I take very seriously and want to continue to honour him and be the best police officer I can," he said. The 29-year-old wears his father's badge number and even has joined the New York Police Department lacrosse team, which his dad founded.
On Sept. 11, 2001, al Qaida extremists hijacked four commercial jets and crashed them into New York's World Trade Center, the Pentagon and a field near Shanksville, Pennsylvania.
The impacts brought down the World Trade Center's twin towers — icons of US economic power, and among the tallest buildings on Earth — and blew a hole in the Pentagon, the US military's fortresslike headquarters near Washington.
The fourth plane also was headed for the nation's capital, officials concluded, but the aircraft went down in Pennsylvania after passengers and crew members stormed the cockpit and fought the hijackers.
"In the course of history, there are a few moments that divide time into, really, what came before" and after, President Donald Trump said at a 9/11 remembrance in Washington this week.
The Republican, who was in his native New York on Sept 11 and has often recounted his memories, plans to attend the Pentagon's anniversary ceremony Friday.
Vice President JD Vance is expected at the observance at the National Sept. 11 Memorial in New York, while other administration officials are headed to the Flight 93 National Memorial in Pennsylvania.
Traditionally, there are no political speeches at the New York commemoration, where relatives read all the victims' names, punctuated with moments of silence at key points in the attack timeline.
For the first time, this year's ceremony will add a silent moment to honour people who were exposed to the clouds of toxic dust released by the twin towers' fiery collapse and have since died of cancer and other illnesses. Questions remain about how many people developed health problems linked to the dust.
Sept 11 generated a wave of patriotism that propelled some Americans to join the military, fire departments, police forces or other public agencies. But amid the national unity that many remember, the attacks also fanned mistrust, prejudice and hate crimes directed at Muslims.
The day isn't a federal holiday, but Congress has designated it both Patriot Day and a National Day of Service and Remembrance.
"One appropriate way to remember is by serving," says Jay Winuk, who lost his brother Glenn Winuk and then co-founded the volunteer organisation 9/11 Day with friend David Paine.
The group helps people find meal-packing and other volunteer opportunities and this year arranged for dozens of Times Square billboards to pause their advertising at 9:11 am Friday.
