Image source, Getty Images

It has been a day filled with painful memories for thousands in the United States and around the world.

Two decades ago, hijackers seized three passenger planes and crashed them into the World Trade Center in New York City and the Pentagon in Washington DC. A fourth hijacked plane, believed to be intended for the US Capitol, crashed into a Pennsylvania field thanks to the heroic efforts of its passengers.

In all, nearly 3,000 lives were lost.

The formal ceremonies in honour of those who died in the carnage or while trying to save others caught up in it have now concluded.

Across the nation, Americans continue to commemorate this 20th anniversary and commit anew to "never forget" the victims.

Thank you for joining our live coverage.

This coverage was brought to you by Holly Honderich, Sam Cabral and Joshua Cheetham, Ritu Prasad and Tom Geoghegan.

Our penultimate post is best expressed by the words of photojournalist David Handschuh who survived the collapse of the Twin Towers. He told the BBC's Radio 4 Today programme how 9/11 changed him.

"I’m very, very thankful to be alive. I get out of bed every morning and open my eyes and I have some aches and pains and I have some memories lodged in my head that might not be the nicest of memories. But I am here.

"No matter how broken I was, I’m remarkably fortunate to, for some reason, still be here today to complain about how my legs hurt, or that I get winded walking a block.

"You have to appreciate every moment. It can be taken away as quickly as you got it. It shouldn’t take a terrible tragedy like 9/11 or any natural disaster for us to realise that we have one life.

"Don’t sweat the little things and don’t forget to say ‘I love you'."

Laura Trevelyan
BBC World News America presenter

Image source, Getty Images

Appeals to revive the spirit of unity which Americans felt in the immediate aftermath of 9/11 have been a striking component of today’s 20th anniversary commemorative events.

President George W Bush evoked the spirit of passengers aboard Flight 93, who overpowered the hijackers, as an example of unity despite their differences.

Confronting extremism at home is our duty, said the former president, who has made no secret of his dislike for the politics of Donald Trump.

Bush used the platform of the 20th anniversary to appeal to Americans to rise above what he called the politics of fear and division.

This came as Trump, who was not present for the formal ceremony marking the anniversary, instead appeared in New York briefly, where reporters peppered him with questions about whether he intends to run again.

He later made an appearance to visit New York firefighters and police.

The contrast between the last two Republican presidents could not be clearer: Bush confronting the shadow of 6 January and the rioters who tried to overturn a presidential election believing it to have been stolen, while Trump, whose style of populism has been embraced by the Republican base, teased another White House run.

Image source, Getty Images

As we reported earlier, former President Donald Trump did not attend today's commemoration ceremony. Instead, he visited emergency service workers in New York City separately.

He gave speeches during visits to a police station and fire station.

Asked by one police officer if he intended to run for a second term, Trump said: "We're not supposed to be talking about it yet... But I think you're going to be happy."

Later today, the former Commander-in-Chief will be providing commentary for a boxing match in Florida, together with his son Donald Jr.

President Biden has just spoken to reporters ahead of his departure from Shanksville to Washington, where he's expected to visit the Pentagon.

"These memorials are really important, but they’re also incredibly difficult for the people affected by them, because it brings back the moment they got the phone call, it brings back the instant they got the news, no matter how years go by," Biden said.

Image source, National Geographic

Joseph Pfeifer, Battalion Chief of the New York City Fire Department, told the BBC about his experiences on that day. He first featured in the National Geographic documentary 9/11: One Day in America

"We were attending a routine gas emergency in the area. And at 08:46 we heard the loud noise of a plane coming overhead, and then I saw the plane aim and crash into the World Trade Center. I was the first fire service commander on the scene.

"Every first responder, when we saw the burning towers, knew this was the most dangerous fire of our lives. Everyone made their own personal decision to go in.

"I thought I might die when the North Tower collapsed. It was so confusing that we didn’t know at that point the South Tower had gone down already. But at that moment someone yelled, ‘the building is collapsing,’ and I started to run. You don’t run too far with helmets and bunker coat and pants and boots, in 11 seconds.

"We wound up ducking behind a small van, and then this beautiful summer day goes completely dark, where you can’t see anything. We hear steel crashing all around us, and glass and then things go quiet. And in the darkness, I wondered if I was still alive.

"My brother Kevin was a lieutenant in the fire service, and that day we stood just maybe a meter apart and we looked at each other. He didn’t say a word, it was just a quiet moment for a few seconds, and then I ordered him up to evacuate the building and to rescue those that were trapped.

"That was the last time I saw him."

Image source, Getty Images

Image source, Getty Images

It is incredible to think that, 20 years on, some 9/11 victims are still being identified.

Dorothy Morgan had been working as an insurance broker at the World Trade Center on the day of the attack and has been missing ever since.

This week she was identified as the 1,646th victim. Another man has been identified as the 1,647th.

Advances in DNA analysis has made it possible for New York City forensic examiners to match bone fragments - in some cases as small as a Tic-Tac candy - to victims thought to have been killed.

Around 40% of the 2,753 who died there are still unidentified. Medical examiners continue to examine over 22,000 pieces of human remains for DNA that matches the body part with the missing person.

"No matter how much time passes since September 11, 2001, we will never forget,” said Dr Barbara Sampson, New York City's Chief Medical Examiner. “We pledge to use all the tools at our disposal to make sure all those who were lost can be reunited with their families."

Mina al-Lami
Jihadist Media Specialist, BBC Monitoring

Image source, Getty Images

Jihadists are celebrating the 20th anniversary of 9/11 as a double victory. It coincides with the Taliban's return to power in Afghanistan, seen as a coup for jihad, and the departure of all US troops from the country.

Al-Qaeda has been at the forefront in congratulating the Taliban on their "historic victory", saying it foreshadows jihadist gains elsewhere.

The anniversary also comes at a time when Western countries are reducing or ending their military presence in conflict zones in Muslim-majority countries - a move jihadists have been pushing for and are now watching with anticipation.

In recent years, al-Qaeda has suffered successive blows to its leadership and operations in several regions, but it is far from eliminated as a serious jihadist threat.

Al-Qaeda, which was linked to the Taliban through an old pledge of allegiance, will no doubt be looking to make gains on the back of the Afghan group’s return to power.

But it is currently unclear, perhaps even to jihadists, what level of support they may or may not get from the Taliban.

After all, al-Qaeda has much to gain from its alliance with the resurgent Taliban, but the Afghan group potentially has much to lose if it again harbours hardline jihadists.

Max Giaccone was 10 when a teacher took a call at school before telling him to go to the school office. As he walked down the corridor, he tells the BBC, he saw his mum, who told him what happened at the World Trade Center where his dad worked.

I had to be sat down and told Dad is not coming home.You know, as a 10 year old, you see on TV that there are people walking over the George Washington Bridge and you know, cell phones were not rapidly available. And cell service was down, and you just hold on to that sliver of hope that he's gonna come home.

And then we got to a point where I think I just finally accepted it. But it took, it took a little while.

I played a lot of baseball, I looked for my dad, everywhere, after I went right back into it about two weeks later. I thankfully had my grandfather and my uncle and my mom and whoever was there, but it's not my dad. And I missed that a lot.

And how does Max feel that loss today, on the anniversary?

I know that 20 years is like a big thing. But to me, it's just another day. It's another day without my dad.

Image source, Max Giaccone

Anthony Zurcher
BBC News, White House

Image source, Anthony Zurcher

Twenty years ago, the White House may have been the target for the hijacked Flight 93, which crashed in Pennsylvania.

Administration staff and the rest of the compound’s occupants scrambled to find shelter, fearful that a devastating attack would happen at any moment.

Today, it’s quiet here.

The president spent the night in New York and will go straight to Delaware after laying a wreath at the Pentagon this afternoon.

Reporters and Secret Service officers are the only visible sign of activity. Outside the gates, tourists pose for photographs.

The US flags flapping at half-mast in the light breeze are the sole reminder of the day’s significance - and the tragedy that may have been narrowly avoided here two decades.

Jon Sopel
BBC North America Editor

Image source, Getty Images

On a stunning, gin-clear September morning they gathered in solemnity and sadness in lower Manhattan: the president and two of his predecessors alongside ordinary New Yorkers who lost loved ones. The weather was identical to that fateful Tuesday morning 20 years ago. But everything else was so different.

It’s hard not to be moved by the haunting and harrowing recitation of the names of those who perished. Today Americans have once again come together - something they seem to find increasingly difficult to do; something that happens increasingly rarely.

President Biden has urged people to rediscover the bonds that tie the citizens of this land together. That is what makes the United States strong, he said in a video the White House released yesterday.

And former President George W Bush made a similar point while speaking at Shanksville, Pennsylvania, decrying the divisive politics of today, contrasting it with the unity of the past.

Twenty years after 9/11, America doesn’t feel that strong.

America is still a superpower – undoubtedly – but doesn’t seem sure what to do with that power. To engage with the world, or try to hide from it?

Twenty years ago it was clear what America had to do: remove the Taliban from power after they’d harboured the al-Qaeda terrorists. Two decades later and the Taliban are forming a new government, after the US withdrawal - a vivid and uncomfortable symbol of American uncertainty.

Our Chief International Correspondent Lyse Doucet has followed every twist and turn of the Afghan story since the war began two decades ago in the wake of 9/11.

In this landmark series of interviews for the BBC World Service, she hears from Afghans how the last 20 years have shaped them: their dreams for a new future and their fears that the cycle of violence will never stop.

Listen wherever you get your podcasts. In the UK you can also find it on BBC Sounds.

Dan Thomas
BBC News, Ground Zero, New York City

Image source, Dan Thomas

Patricia, 48, knew two people who died at the World Trade Center on 11 September.

Her cousin Brian, a fireman, was on his way home when he was called back to help evacuate the towers.

Her friend Jason worked for financial services firm Cantor Fitzgerald at the top of One World Trade Center. All of the company’s employees who reported for work that morning were killed.

Patricia, who herself worked just three blocks from Ground Zero and was evacuated by ferry that day, says this is the first 9/11 memorial service she’s been to as "there are too many memories being here".

“It was shocking, scary, unknown," she says. "I was on the phone when the first tower came down and on a boat for the second."

However, she is glad she made it this time: "My sister’s reading my cousin’s name so I’m escorting her."

Image source, Getty Images

During 9/11, Daniel Nigro was Chief of Operations at New York City's Fire Department (FDNY), and the second in command at Ground Zero.

His superior and friend of 30 years, Chief Peter Ganci Jr, was killed when the North Tower came down.

Daniel became the department chief and has held every rank in the FDNY in a career spanning half a century.

He told BBC Radio 5 Live that every year, when August comes around, "anxiety starts to build up".

"I tell people it’s like when you receive a wound and it heals but it leaves a scar," he said.

"I have the scar but I’ve been able to enjoy my life for these past 20 years and I’m extremely thankful for that."

He still remembers the events of 9/11 vividly.

“The day has not faded at all," he said.

"People needed us. The calls that were coming into our dispatch office... I’ve heard them since and they’re awful to hear. People were calling for help and all we could say was ‘we’re coming, we’re coming,’ but we couldn’t get up to those floors in time."

Members of a military band played Taps to mark the conclusion of what has been an emotional ceremony for the families of those who lost their lives.

Nearly 3,000 names were read in all - the 2,983 victims of the 2001 attacks in addition to six people who died at the 1993 bombing of the World Trade Center.

Image source, Getty Images

As the US marks 20 years since 9/11, memorial ceremonies have been taking place at the sites of the attacks in New York City, Washington DC and Shanksville, Pennsylvania.

Here are some of the day's key moments:

• At Ground Zero, where the Twin Towers once stood, guests - including President Joe Biden and other political leaders - observed a minute of silence at 08:46 local time, the moment when the first plane crashed into the North Tower.
• Five more moments of silence, in the three different cities, have since followed, marking the three other plane crashes and the collapse of the World Trade Center buildings.
• In a poignant ceremony in New York, family members of the 2,977 people who died that day have just finished reading every name - at times reminiscing about who they were, why their lives mattered and how much they are missed.
• Bruce Springsteen and other musicians sang to the crowd assembled at the memorial ceremony in New York.
• Former President George W Bush and Vice-President Kamala Harris were among those who paid tribute in Shanksville to the heroic passengers of Flight 93, who prevented their hijackers from taking control of the plane.
• President Biden made no formal remarks today but has laid wreaths at Ground Zero and in Shanksville. He will also lay a wreath at the Pentagon building, the site of the third plane crash.
• Later tonight, New York will hold a tribute in light to the fallen victims.

George Bush's national security adviser at the time of the attacks speaks to the BBC about that day, and the withdrawal from Afghanistan.

Then a fifth-term US Senator and the head of the chamber's Foreign Relations Committee, he first learned of the attacks while on his usual Amtrak train ride to Washington DC.

By the time he reached the Capitol building, the Twin Towers in New York and nearby Pentagon were ablaze.

That afternoon, Biden called then-President George W Bush. They spoke for two minutes, records show. Biden would later tell CNN he felt the way Bush "handled events from that moment until we took down the Taliban was textbook".

The day after, Biden delivered an emotional speech on the floor of the Senate, promising that America's enemies "will not, cannot defeat us".

Image source, Getty Images

This is the president's second 9/11 memorial event of the day, in Pennsylvania. He heads to the Pentagon later.

Image source, Getty Images

When United Airlines Flight 93 crashed into that remote field in Pennsylvania, Gordon Felt lost his brother Edward.

A 41-year-old engineer and father of two, Edward Felt was seated in the second row of the plane's first class section - directly in front of two of the hijackers.

When the attackers took over, he reportedly made a 911 emergency call from his cell phone to report the hijacking in progress.

He is believed to be among the many passengers who fought back and prevented the plane from making its way to Washington DC.

Gordon Felt has served as the president of "Families of Flight 93", a non-profit that sustains the permanent memorial to his brother and the 39 other passengers killed in the crash.

In a poignant address at today's memorial ceremony, he paid tribute to their "extraordinary" sacrifice but wondered aloud where the nation's unity had gone.

"Are we worthy of their sacrifice?" he asked. "As a country, do we conduct ourselves in a way that would make them proud?"