The September 11 anniversary commemoration at ground zero in New York City began with a tolling bell and a moment of silence, exactly 20 years since the start of the deadliest terror attack on US soil.
US president Joe Biden was joined by former presidents Barack Obama and Bill Clinton and a crowd of victims’ relatives and first responders for the ceremony at the 9/11 memorial plaza in the city.
The memorial stands where the the World Trade Centre’s twin towers were rammed and felled by hijacked planes.
Observances are also planned at the the two other sites where the 9/11 conspirators crashed the jets: the Pentagon, and a field near Shanksville, Pennsylvania. Mr Biden is scheduled to pay respects at all three sites.
The anniversary arrives under the shadow of the coronavirus pandemic and the US withdrawal from Afghanistan – now ruled by the same militants who gave safe haven to the 9/11 plotters.
The US attacks killed nearly 3,000 people and marked the start of a new era of fear, war, politics, patriotism and tragedy.
In a video released on Friday night, Mr Biden mourned the ongoing losses of 9/11.
“Children have grown up without parents, and parents have suffered without children,” said Mr Biden, whose childhood friend is the father of a September 11 victim, Davis Grier Sezna Jr.
But the president also underlined what he called the “central lesson” of September 11: “That at our most vulnerable … unity is our greatest strength.”
Former president George W Bush, America’s leader on 9/11, is due to attend the Pennsylvania memorial.
The only other post-9/11 US president, Donald Trump, plans to be in New York, in addition to providing commentary at a boxing match in Florida in the evening.
Other observances – from a wreath-laying in Portland, Maine, to a fire engine parade in Guam – are planned across a country now full of 9/11 plaques, statues and commemorative gardens.
Using hijacked planes as missiles, the assailants ushered in an age of fear by toppling the twin towers – an event people across the world saw unfold on live television.
Security was redefined across the west, with changes to airport checkpoints, police practices and the surveillance powers of governments.
In the years that followed, virtually any sizeable explosion, crash or act of violence seemed to raise a dire question: “Is it terrorism?”
Some ideological violence and plots did follow, though US security services and the public have lately become increasingly concerned with threats from domestic extremists after years of focusing on international terror groups in the wake of 9/11.
New York faced questions early on about whether it could ever recover from the blow to its financial hub and restore a feeling of safety among the crowds and skyscrapers.
New Yorkers ultimately rebuilt a more populous and prosperous city, but had to reckon with the tactics of an empowered post-9/11 police department and a widened gap between haves and have-nots.
The “war on terror” led to invasions of Iraq and Afghanistan, where the longest US war ended last month with a hasty mass airlift punctuated by a suicide bombing that killed 169 Afghans and 13 American service members, attributed to a branch of the so-called Islamic State extremist group.
US authorities are now concerned that al Qaida, the terror network behind 9/11, may regroup in Afghanistan.
Two decades after helping to triage and treat injured colleagues at the Pentagon on September 11, retired Army Col Malcolm Bruce Westcott is saddened and frustrated by the continued threat of terrorism.
“I always felt that my generation, my military cohort, would take care of it – we wouldn’t pass it on to anybody else,” said Col Westcott, of Greensboro, Georgia. “And we passed it on.”
For Angelique Tung, who was at the trade centre for a business meeting on 9/11 and escaped down 77 flights of stairs, the US pullout from Afghanistan stirred empathy for troops who served there.
Some now wonder whether their efforts and sacrifices made a difference, which makes Ms Tung think of a question she has asked herself since surviving September 11.
“I hope that, after 20 years, other people are asking that question: What good can come from this?”
September 11 propelled a surge of shared grief and common purpose, but it soon gave way.
Muslim Americans endured suspicion, surveillance and hate crimes. The quest to understand the catastrophic toll of the terror attacks prompted changes in building design and emergency communications, but it also spurred conspiracy theories that seeded a culture of scepticism.
Schisms and resentments grew over issues such as immigration, the balance between tolerance and vigilance, the meaning of patriotism, the proper way to honour the dead, and the scope of a promise to “never forget”.
Thea Trinidad was 10 when she overheard her father, Michael, saying goodbye to her mother by phone from the burning trade centre.
As she prepared to read victims’ names at the ceremony at ground zero in New York, she said: “It’s hard, because you hoped that this would just be a different time and a different world. But sometimes history starts to repeat itself and not in the best of ways.”
Ms Trinidad remembers the pain but also the fellowship of the days that followed, when all of New York “felt like it was family”.
Ms Trinidad, who now lives in Orlando, Florida, said: “Now, when I feel like the world is so divided, I just wish that we can go back to that.
“I feel like it would have been such a different world if we had just been able to hang on to that feeling.”