September 11 attacks marked by commemoration ceremonies, 20 years on
Thousands have gathered in New York and across the United States for ceremonies commemorating the 20th anniversary of the September 11 attacks.
Memorials were held in New York City, the Pentagon, and Pennsylvania — all sites where hijacked planes were crashed in a coordinated Al Qaeda attack 20 years ago.
A moment of silence at 8:46am local time (10:46pm AEST) was followed by a tolling bell to begin the program in New York City.
It marked the exact time on September 11, 2001, when American Airlines flight 11 crashed into the North Tower of the World Trade Center.
Americans are honouring the nearly 3,000 lives lost in the attacks, while reflecting on how they shaped the country's view of the world and itself.
US President Joe Biden traveled to all three sites of the attacks.
He began the day in New York at the first ceremony.
Mr Biden and former presidents Bill Clinton and Barack Obama held a moment of silence among a crowd of victims' relatives and first responders.
Mike Low, who lost his daughter Sara Elizabeth Low in the New York attack, addressed the large crowd after the initial moment of silence.
"These 20 years have felt like a long time and a short time and as we recite the names of those we lost, my memory goes back to that terrible day when 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," Mr Low said.
The reading of the names was stopped at 9:03am local time for another moment of silence to mark when hijacked Flight 175 struck the South Tower.
Music legend Bruce Springsteen performed I'll See You In My Dreams before the names of victims continued to be read by loved ones.
Mr Biden then travelled to Shanksville, Pennsylvania, where United Flight 93 crashed into a field after passengers overcame the hijackers and prevented another target from being hit.
Harris, Bush speak in Pennsylvania
US Vice President Kamala Harris and George W Bush, who was president from 2001-2009, spoke at the memorial for the 33 passengers and seven crew of United Flight 93 that crashed near Shanksville, Pennsylvania.
The audience included Mr Bush's former vice president Dick Cheney.
"Twenty years ago we all found in different ways in different places, but all at the same moment, that our lives would be changed forever," Mr Bush said.
"The world was loud with carnage and sirens and then quiet with missing voices that would never be heard again."
Striking a tone of unity, Mr Bush also spoke about what the attack revealed about America's character as he saw it:
"At a time when religious bigotry might have flowed freely, I saw Americans reject prejudice and embrace people of the Muslim faith – that is the nation I know," he said.
Ms Harris also struck a note of unity as she commemorated those onboard the flight, and the first responders.
"What happened on Flight 93 told us then and it still tells us so much about the courage of those on board, who gave everything they possibly could. About the resolve of the first responders, who risked everything," she said.
"On the days that followed September 11th, 2001, we were all reminded that unity is possible in America. We were reminded also that unity is imperative in America. [...] And by unity I don't mean uniformity.
Commemorations at the Pentagon
Defense Secretary Lloyd Austin said "we remember not just who our fallen teammates were, but the mission that they shared" in prepared remarks at the Pentagon ceremony.
He noted that "almost a quarter of the citizens who we defend today were born after 9/11", including many of the 13 American service members killed in the recent attack in Afghanistan.
"We cannot know what the next 20 years will bring. We cannot know what new dangers they will carry … but we do know that America will always lead."
The losses of 9/11
In a video released on the evening of September 10, Mr Biden mourned the ongoing losses of 9/11.
"Children have grown up without parents, and parents have suffered without children," he said.
But the president also spotlighted what he called the "central lesson" of September 11 "that at our most vulnerable … unity is our greatest strength."
The only other post-9/11 US president, Donald Trump, plans to be in New York, before returning to Florida on Saturday evening.
The anniversary takes place just weeks after the chaotic US withdrawal from Afghanistan and the return to power of the Taliban.
The Taliban is the faction that sheltered the Muslim militant group founded by Osama bin Laden, which carried out the attacks.