Image source, Getty Images

As President Biden heads to Shanksville to pay tribute to those who perished on the crashed aircraft, there's more to say about what happened to that flight - or could have happened.

Minutes after the two planes struck the World Trade Center, then Vice-President Dick Cheney was rushed to a special operations bunker within the White House.

With President George W Bush evacuated into the skies on Air Force One, Cheney found himself surrounded by senior advisers with the mission of preventing further attacks.

As detailed in journalist Garrett Graff's oral history of the day, the Pentagon asked for permission to shoot down an additional hijacked aircraft - United Airlines Flight 93.

"Without hesitation", Cheney confirmed the unthinkable - that a plane filled with Americans could be shot down, according to Graff's reporting.

Cheney later said: "It had to be done." His order was then approved by President Bush.

But in the end, no military pilots ever had to carry out the order.

The passengers of Flight 93 managed to overpower the hijackers and crashed the plane into a field near Shanksville, Pennsylvania. All 44 on board lost their lives.

Cheney spoke to the BBC about this in a documentary about 9/11, which is linked below

9/11: Inside the president's war room

Image source, Bilal Sarwary

Afghan journalist Bilal Sarwary watched his country change in the 20 years since the 9/11 attacks.

In 2001, I was a carpet salesman at the Pearl Continental Hotel in Peshawar, Pakistan, having yet another unremarkable day at work.

I'll never forget glancing up at the TV in a brief moment between sales, only to witness firsthand the dramatic footage as a passenger plane careered into the World Trade Center in New York. Then the second plane, and another at the Pentagon.

None of our lives would ever be the same.

The Americans came, and the Taliban was toppled.

Their fighters vanished into the mountainous rural areas, and their leadership fled to Pakistan.

In retrospect, it is clear to me that this was a missed opportunity, a time when the US should have sat down with the Taliban to discuss a peace deal. I saw a genuine willingness amongst the rank-and-file of the Taliban to lay down their arms and resume their lives.

But the Americans didn't want that. From my reporting, it seemed to me and many other Afghans that their motivation was revenge after 9/11.

The ensuing years were a catalogue of errors.

Read more from Bilal

Gary O'Donoghue
BBC News, Pentagon, Virginia

On a day when ritual and remembrance are central, it is particularly striking that former President, George W Bush, chose a high profile appearance at the site of Flight 93's crash to bemoan the nature of the current discourse in America.

Not only did he echo Joe Biden and others in mourning the passing of national unity, but he also drew a direct connection between those who would bring violence from abroad and those who foment it at home. They are, he said, "the children of the same foul spirit".

There will be few who won't see that as some kind of reference to the violence we have seen around last year's election and the storming of the Capitol.

But he didn't stop there; he talked of America's politics being fuelled by anger, fear and resentment - something that has come to dominate political argument in this country.

Of course, Bush's dislike for Donald Trump and the new Trumpian Republican party is no secret, but this intervention on a day like this supercharges the criticisms and will no doubt draw some sharp responses.

Dan Thomas
BBC News, Ground Zero, New York City

Roland Gillis’s brother Rodney was a New York City police officer who died trying to help evacuate the South Tower of the World Trade Center before it came down.

Rodney’s last conversation with the family was with their mother on 11 September, before he made his way to the tower.

"He said 'I love you, I’ll see you soon.'"

Roland says his brother "believed in life and liberty and family being together".

"He thought people should just live together [in harmony] but we live in a world that’s not like that."

He says he misses "everything" about his brother who was a character and liked to joke around.

"But the thing I miss most is that he hasn’t been able to raise his kids. And now he has four grandkids and I’m there for them."

Frank Gardner
BBC Security Correspondent

Image source, Getty Images

What lessons, if any, have been learned from the 20 years since 9/11 of fighting terrorism across the world?

1. Share vital intelligence. Since 9/11, the US and UK have made huge changes to how information is gathered and shared among intelligence agencies and other government departments.
2. Define the mission and don't get distracted. Of all the many reasons why Afghanistan has reverted to Taliban rule, one stands out: the US-led invasion of Iraq in 2003. This ill-fated decision became a massive distraction to what was going on in Afghanistan.
3. Choose your partners carefully. During the 2003 Iraq invasion, Britain was a junior partner in nearly all key decisions. The collective panic after 9/11 also meant US and British intelligence ended up cooperating with regimes with appalling human rights records.
4. Respect human rights or lose the moral high ground. Time and again people in the Middle East have told me: "We may not have liked US foreign policy but we always respected its rule of law. Until Guantanamo Bay."
5. Have an exit plan. The western interventions that preceded 9/11 were relatively quick and simple by comparison. Sierre Leone, Kosovo, even the Desert Storm campaign of 1991 - all had a finite ending. But the US-led invasions of Afghanistan and then Iraq have resulted in what have been called "the forever wars".

Read the full story here.

On 11 September 2001, Stephen Evans was a BBC reporter heading to an 09:30 meeting in the World Trade Center. In a fortunate twist, he changed his mind about having breakfast up there and instead stopped for coffee down the street before entering the ground floor of the South Tower.

"At 08:46 there was an almighty, metallic crash. Dust filled the lobby and I got out. Above me, I could see the gash where the first plane had gone in. At 09:00 I was on the air [broadcasting for the BBC] in a shop at the base of the tower," he recalled.

"Then the second plane went in and the newsagent pulled down the shutters. I hired a room at the Embassy Suites hotel just for the phone. When the tower collapsed I was evacuated down the backstairs, the alarm blaring. I was in the street when the South Tower collapsed. It was the only moment of true, in-your-guts, fear.

"The debris exploded towards me and I thought ‘that’s coming at me faster than I can run.’ Fortunately, I was wrong."

Evans later shared a taxi with a Chinese American woman who said she was going into labour and gave him a piece of paper with her number on it, to tell her husband she was OK.

He also met a widow in inconsolable grief because her husband was trapped. The two of them spent an hour and a half on the phone - when the tower collapsed, the phone went dead.

Laura Trevelyan
BBC World News America presenter

Image source, Aiden Johnson

Brenda Berkman, a former captain in the New York fire department who rushed to the scene 20 years ago, tells me she still has survivor's guilt.

Brenda knew more than 200 of the 343 firefighters killed that day.

Her life was full of funerals and grief for months, and the memory of the day itself is seared into her consciousness forever.

An inferno greeted Brenda and her off-duty colleagues as they raced to the scene to see the North Tower of the World Trade Centre collapse. It was the most horrific sight any firefighter had ever experienced.

Brenda has turned to art as she continues to process what happened and has dedicated herself to ensuring the role of female first responders is included in the history of 9/11.

Image source, Getty Images

We heard moments ago from Vice-President Kamala Harris in Shanksville.

She said the events that unfolded on Flight 93 "tells us so much about the courage of those on board, who gave everything...about the resilience of the American people".

Harris hearkened back to the period of national unity that followed 9/11.

"In a time of outright terror, we turned toward each other. In the face of a stranger we saw a face of a friend," she says. It "reminded us of the significance and the strength of our unity as Americans, and that it is possible in America".

"By unity, I don't mean uniformity. We had differences of opinions in 2001 as we do in 2021," she added. "I believe in America, our diversity is our strength."

We've just heard former President George Bush acknowledge the heroism of the passengers on United Airlines Flight 93, which crashed near Shanksville, Pennsylvania, after they tried to seize control of the plane.

On voice recordings recovered from the flight, one of the hijackers can be heard speaking to passengers over the intercom.

"We have a bomb aboard. We're going back to the airport. We'd like you to sit down and remain quiet."

But famously, the passengers did not sit down and remain quiet.

Through their phones, they learned of the three planes that had just crashed into the World Trade Centre and the Pentagon.

They made a desperate calculation - if they did not fight back, they would die.

The recordings revealed the moment one of the hijackers issued an order to cut off the oxygen. Crashing sounds followed, alarms rang, and, with a tremendous screech, the plane rolled over.

The plane pitched and swung madly before it crashed into the remote field on the outskirts of Shanksville.

All 44 people on board were killed.

Image source, Getty Images

Image source, Getty Images

More now from former President George W Bush in Shanksville.

He recalls the day when American lives were changed forever.

"We share your sorrow and we honor the men and women you have loved for so long and so well," Bush said.

Addressing those too young to remember the day, Bush said it is "hard to describe the mix of feelings we experienced".

"Shock at the audacity of evil, and gratitude of the heroism and decency that opposed it."

He continued: "The world was loud with carnage and sirens, and then quiet with missing voices, never to be heard again."

The former president made an apparent reference to the wars in Afghanistan and Iraq, as he acknowledged that the "military measures" that followed 9/11 "have led to debate".

He then addressed the veterans of these battles directly.

"The cause you pursued at the call of duty is the noblest America has to offer," Bush said.

“Nothing that has followed, nothing, has tarnished your honour."

Image source, Getty Images

In the aftermath of 9/11, one of the most iconic images emerged from an impromptu speech given by then-President George W Bush, during an appearance at Ground Zero.

Bush visited the site on 14 September with rescue workers, firefighters and police officers.

Standing on a pile of rubble, next to retired firefighter Bob Beckwith, the Republican president went on to give his so-called "bullhorn address" - a speech that's gone down in American history.

"I want you all to know that America today, America today is on bended knee, in prayer for the people whose lives were lost here, for the workers who work here, for the families who mourn," Bush said, megaphone in hand.

As he continued his speech, people nearby yelled back, "We can’t hear you!"

"I can hear you! I can hear you! The rest of the world hears you," replied Bush. "And the people who knocked these buildings down will hear all of us soon."

Bush spoke to the BBC about the speech in a documentary about 9/11, which is linked below.

9/11: Inside the president's war room

Image source, Getty Images

Former president George W Bush is now speaking at the memorial ceremony in Shanksville, Pennsylvania.

He was leading the country when the attacks happened and he signed the legislation that created a memorial at this site.

Aleem Maqbool
BBC North America correspondent

This grim anniversary has renewed focus on the five suspects in detention accused of involvement in the 9/11 attacks.

The men have all appeared in court in Guantanamo Bay this week after an 18-month hiatus in pretrial hearings caused by the pandemic.

In the viewing gallery, observing these proceedings, there have been a small number of relatives of victims, some members of not-for-profit organisations and a handful of journalists.

Guantanamo Bay already feels cut off from the world and given the magnitude of this case and weight of the horrific crimes being considered the courtroom here feels all the more singularly alien.

"Walking into the courtroom for the first time was extremely emotionally powerful for me," says Dr Elizabeth Berry, whose younger brother Billy Burke was one of the firemen in the North Tower when it collapsed.

"I wasn't quite sure what to expect because you see things in the newspapers, portrayals of the way people look which are not really an accurate representation when you see them in the courtroom. It was very moving and very difficult," she says.

Dr Berry has attended many of the 42 pretrial hearings in this case at Guantanamo Bay and says she specifically wanted to be here for the 20th anniversary of the attacks to feel she was supporting the team fighting for justice for her brother and nearly 3000 others.

Read more about the proceedings in Guantanamo Bay

As commemorations continue stateside, leaders from around the world have been paying their respects online.

In a tweet, French President Emmanuel Macron shared a video of an American flag at the Élysée Palace, with the caption: "We will #NeverForget. We will always fight for freedom."

Ursula von der Leyen, president of the European Commission, also paid tribute to "those who lost their lives and... those who risked everything to help them".

"Even in the darkest, most trying of times, the very best of human nature can shine through,” she tweeted.

Australian Prime Minister Scott Morrison has published a lengthy opinion piece, reflecting on the legacy of 9/11.

"That day was an attack on free peoples everywhere. It was an attack on our way of life and the values of liberal democracy," Morrison wrote.

"Despite the pain inflicted on that day, the terrorists ultimately failed in their attempts to crush our resolve and change our way of life," he added.

"September 11 reminded us that freedom is always fragile."

Maxine Hughes
BBC News, New York City

Image source, Maxine Hughes

As the first moment of silence began to mark the moment the first plane hit the North Tower, people gathered on Greenwich Street and looked directly up at One World Trade Center.

Many were New York firefighters.

A man quietly whispered to his young child.

An elderly man wiped a tear.

A few minutes later, a local New York reporter tells me that this was the moment, 20 years ago, that his network started rolling news, and didn’t run a single commercial for three weeks.

Another, final moment of silence is being held to mark the collapse of the World Trade Center's North Tower, at 10:28 local time (15:28 BST).

Nomia Iqbal
BBC News, New York City

Image source, Nomia Iqbal

Many New Yorkers are trying to get on with everyday life - jogging, cycling and walking their dogs. Some stop occasionally to take in the huge lines of police cars that form a barricade around Ground Zero, where the ceremony is taking place.

Some here are also from abroad. Lola, Soundarya and Marie are all friends from France, working in New York.

They’re also connected by the fact they have no memory of 9/11 - they were all just one year old.

Soundarya says: "This is my first time here when the anniversary is happening. It’s so surreal because I learned about it in documentaries and watching news coverage."

The question often asked is: Where were you on that day?

Marie says she wanted to be here so she can remember in her own way.

"Growing up, I always heard it from other perspectives, other people’s experiences. Being here means I can watch it and create my own moment to mark the anniversary."

Image source, Getty Images

At the ongoing ceremony in New York City, we're hearing from Anthoula Katsimatides as she remembers her brother, John.

She recalls the dark days that followed the attacks, when she and thousands of others joined a "club" they had never wanted to be a part of - a club of those mourning their missing loved ones.

Katsimatides remembers an outpouring of support. "The whole country put its arms around us," she said. "And all this kindness and giving reminded me of my brother."

Katsimatides' eyes glisten with tears, but she speaks with a slight smile.

In the past 20 years, she said, she has worked to emulate him, her "larger than life big brother".

"I know it's made me a better person," she said. "I came here, yes, with tears, but also with a really happy heart."

President Joe Biden is attending official commemoration ceremonies in New York, Pennsylvania and Washington - and several former US presidents are also participating in the tribute events.

President Barack Obama has joined his former vice-president at the New York City memorial. George W Bush will visit the Flight 93 memorial at the Pennsylvania crash site, and is expected to speak.

It’s unclear what 96-year-old Jimmy Carter, the oldest living president, may be doing today.

Earlier this morning, former President Donald Trump released a video statement, praising first responders - and adding it was a "sad time for the way our war on those that did such harm to our country ended last week".

Trump is also doing something quite different later today.

The 45th president and his eldest son, Donald Jr, will be providing commentary for a boxing match in Florida.

"I love great fighters and great fights," the former president said in a statement. "I look forward to seeing both this Saturday night and sharing my thoughts ringside."

The longtime New Yorker, who now calls Florida home, told Fox News that he'll be visiting Manhattan on Saturday - but did not say what he would be doing during his stay. There are reports that he will visit the memorial.