Joe Biden is due this morning to join families of the victims at three separate locations in what he may have hoped would prove a rare moment of national unity. But anger at the US president's recent botched withdrawal from Afghanistan is still raw.
Two decades on, images from 11 September 2001 are vividly imprinted in the memory of some – everyone can remember where they were – but now represent a historical event for a new generation born after the atrocity.
In a crystalline blue sky, al-Qaida terrorists hijacked four commercial airliners and crashed two of them into the World Trade Center, a symbol of America's financial might in downtown Manhattan.
People from all over the world were killed in the initial explosions, jumping to their deaths or being pulverised by the collapsing twin towers, a spectacle that horrified audiences on live television.
The hijackers rammed another plane into the Pentagon, the headquarters of the US military near Washington, tearing a hole in its side. A fourth plane – possibly heading towards the US Capitol – crashed into a field in Shanksville, Pennsylvania, after its passengers heroically fought back.
A total of 2,977 people were killed – 2,753 of them at what became known as "Ground Zero" in New York – a bigger toll than the "day of infamy" at Pearl Harbor in 1941.
Biden and the first lady, Jill Biden, will attend solemn ceremonies at all three sites on Saturday to "honor and memorialize the lives lost", the White House said. At the 9/11 memorial in New York, at 8.30am, they will be joined by former president Barack Obama, who oversaw the killing of the al-Qaida leader Osama bin Laden in 2011.
Biden will travel to Shanksville on Saturday afternoon before visiting the Pentagon for a wreath-laying ceremony. But he is still facing congressional investigations into the chaotic departure of US forces from Kabul, less than two weeks ago, that ended America's longest war.
The Taliban are now back in control of Afghanistan, just as they were on 9/11, raising fears that the country could once again become a terrorist hub.
Former president George W Bush, who launched the retaliatory war in Afghanistan in 2001, will speak at a memorial service in Shanksville. At 10.03am the names of the passengers and crew members, who were all killed, will be read out.
Former president Donald Trump will reportedly visit the 9/11 memorial on Saturday afternoon, once the ceremony has finished and after Biden has left the city.
The memorial includes cascading water in the footprints of the twin towers. At Saturday's solemn ceremony, the names of each victim will be read out with a tolling bell, lasting about four hours.
There will be six moments of silence, marking the times that each tower was struck and the times that each fell, as well as the moment when the Pentagon was attacked, and the time that United 93 crashed in Pennsylvania.
Some services and acts of remembrance were held on Friday. In a ceremony at the state department in Washington, Antony Blinken, the secretary of state, said: "9/11 was, to understate, one of the darkest days in our history – but out of it also came these demonstrations of profound humanity, compassion, strength and courage.
"Above all, it showed our remarkable resilience. It showed our capacity to defend the pluralism that has long been one of our country's greatest strengths, including by embracing our Muslim American brothers and sisters. It showed the risks that so many are willing to take to save the lives of complete strangers. So today, we remember all that, and more."