At sundown, two vertical beams of bright blue light shot upward through the darkened New York City sky, evoking the twin towers in an annual tribute visible for kilometres.
The "Tribute in Light" public art installation first shone six months after the September 11 attacks and has been repeated each anniversary since, with the twin columns reaching up to four miles into the sky from dusk to dawn.
The beams are comprised of 88 xenon light bulbs, each 7000 watts, positioned into two 14m squares on the roof of a parking garage south of the 9/11 Memorial. They can be viewed from a 100km radius.
The 9/11 Memorial and Museum presents the display to honour those who were killed and celebrate the spirit of the city.
For the 20th anniversary, buildings throughout New York also planned to light up their facades and rooftops in blue.
US Presidents pay respect
The world solemnly marked the 20th anniversary of 9/11 on Saturday, grieving lost lives and shattered American unity in commemorations that unfolded just weeks after the bloody end of the Afghanistan war that was launched in response to the terror attacks.
Victims' relatives and four US presidents paid respects at the sites where hijacked planes killed nearly 3000 people in the deadliest act of terrorism on American soil.
Others gathered for observances from Portland, Maine, to Guam, or for volunteer projects on what has become a day of service in the US. Foreign leaders expressed sympathy over an attack that happened in the US but claimed victims from more than 90 countries.
"It felt like an evil spectre had descended on our world, but it was also a time when many people acted above and beyond the ordinary," said Mike Low, whose daughter, Sara Low, was a flight attendant on the first plane that crashed.
"As we carry these 20 years forward, I find sustenance in a continuing appreciation for all of those who rose to be more than ordinary people," the father told a ground zero crowd that included President Joe Biden and former presidents Barack Obama and Bill Clinton.
In a video released Friday night, Biden said September 11 illustrated that "unity is our greatest strength."
Unity is "the thing that's going to affect our well-being more than anything else," he added while visiting a volunteer firehouse on Saturday after laying a wreath at the 9/11 crash site near Shanksville, Pennsylvania. He later took a moment of silence at the third site, the Pentagon.
The anniversary was observed under the pall of a pandemic and in the shadow of the US withdrawal from Afghanistan, which is now ruled by the same Taliban militant group that gave safe haven to the 9/11 plotters.
"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," Thea Trinidad, who lost her father in the attacks, said before reading victims' names at the ceremony.
Bruce Springsteen and Broadway actors Kelli O'Hara and Chris Jackson sang at the commemoration, but by tradition, no politicians spoke there.
At the Pennsylvania site — where passengers and crew fought to regain control of a plane believed to have been targeted at the US Capitol or the White House — former President George W Bush said September 11 showed that Americans can come together despite their differences.
"So much of our politics has become a naked appeal to anger, fear and resentment," said the president who was in office on 9/11. "On America's day of trial and grief, I saw millions of people instinctively grab their neighbour's hand and rally to the cause of one another. That is the America I know."
"It is the truest version of ourselves. It is what we have been and what we can be again."
Calvin Wilson said a polarised country has "missed the message" of the heroism of the flight's passengers and crew, which included his brother-in-law, LeRoy Homer.
"We don't focus on the damage. We don't focus on the hate. We don't focus on retaliation. We don't focus on revenge," Wilson said before the ceremony. "We focus on the good that all of our loved ones have done."
Former President Donald Trump visited a New York police station and a firehouse, praising responders' bravery while criticising Biden over the pullout from Afghanistan.
"It was gross incompetence," said Trump, who was scheduled to provide commentary at a boxing match in Florida in the evening.
The attacks ushered in a new era of fear, war, patriotism and, eventually, polarisation. They also redefined security, changing airport checkpoints, police practices and the government's surveillance powers.
A "war on terror" led to invasions of Iraq and Afghanistan, where the longest US war ended last month with a hasty, massive airlift punctuated by a suicide bombing that killed 169 Afghans and 13 American service members and was attributed to a branch of the Islamic State extremist group.