Image source, Getty Images

An Islamist extremist network called al-Qaeda planned the attacks from Afghanistan.

Led by Osama Bin Laden, they blamed the US and its allies for conflicts in the Muslim world.

Fifteen of the hijackers were Saudis like Bin Laden himself. Two were from the United Arab Emirates, one was from Egypt and one was from Lebanon.

Less than a month after the attacks, President George W Bush led an invasion of Afghanistan - supported by an international coalition - to eradicate al-Qaeda and hunt down Bin Laden.

However, it was not until 2011 that US troops finally located and killed Bin Laden in neighbouring Pakistan.

The alleged planner of the 9/11 attacks, Khalid Sheikh Mohammad, was arrested in Pakistan in 2003. Since then he has been held in US custody at Guantanamo Bay, Cuba, and is still awaiting trial.

US troops left Afghanistan this year after nearly 20 years, stoking fears from many that the Islamist network could make a comeback.

Read more in our explainer about 9/11.

Laura Trevelyan
BBC World News America presenter

The sun is rising over the 9/11 memorial on the 20th anniversary of the attacks. It’s a bright blue, late summer day, eerily similar to the fateful morning in 2001.

Today is a solemn one for New Yorkers, who are remembering the horrifying events of that September day - the overwhelming loss of life, the chaos, the fear, yet also the heroism of the first responders who rushed here, and now the resilience of a city 20 years on.

There will be a citywide moment of silence in New York City at 08:46 local time (13:46 BST). That is the time the first plane hit the World Trade Center.

That will be followed by the reading aloud of the names of all 2,977 victims of the attacks, by bereaved relatives.

Through the morning, the reading of the names will be paused at the moments each plane crashed, and the moments each of the two buildings collapsed.

From west coast to east, people have been sharing private moments, attending memorials and carrying out their own acts of remembrance in the past few days.

In Pennsylvania, people have been visiting the Wall of Names which remembers the passengers of Flight 93 who successfully prevented their hijacked aircraft from being used to attack the US Capitol building in Washington.

Image source, Getty Images

Firefighters at Ladder 15 and Engine 6 station house have been remembering their fallen friends. The New York Fire Department lost 343 firefighters that day, including 14 from this station house on South Street in Lower Manhattan.

Image source, Getty Images

In Malibu, California, they will mark the day with the 14th annual "Waves of Flags" display. The event raises a flag for each life lost and includes a national flag for each foreign nation that lost a citizen during the attacks.

Image source, Getty Images

And 125 people were killed when Flight 77 hit the Pentagon. They have been remembering those who lost their lives with The Tower of Light, which illuminated the Arlington skyline.

Image source, Getty Images

There were 67 Britons killed in the attacks, and memorial events are being held in the UK too, where leaders have been paying tribute.

Prime Minister Boris Johnson said "precisely because of the openness and tolerance of the United States" almost every nationality and religion were among those murdered that day 20 years ago.

Labour leader Sir Keir Starmer said the anniversary was about remembering and honouring those who were killed and showing "support to our American friends as they mark this difficult time in their history".

There were also words of comfort from London Mayor Sadiq Khan, who said the lives of all the innocent people, including the 67 Britons, would be remembered, adding: "Our values of freedom, tolerance and respect will always, always prevail. Hate will never win."

Archbishop of Canterbury Justin Welby said 20 years on it was still hard to articulate the sense of "shock and horror" felt around the world after the attacks - including "the devastating loss experienced by so many people, and the fear and uncertainty that terrible day brought".

Image source, PA Media

Nada Tawfik
BBC News, New York City

The sun is shining and the morning calm is punctuated by the solemnness of this day.

The bagpipes are rehearsing for the processional later with dignitaries and President Biden. On 9/11 anniversaries, New Yorkers are always a bit kinder with each other, a bit gentler.

Twenty years, and perhaps no amount of time, will diminish that connection. There’s a visible police presence around the city.

Ahead of the ceremony today, there’s an intimate vigil under way at the Fire Department of New York memorial wall, a 56-foot-long bronze wall that honors the 343 firefighters who died that day. I wonder what words were spoken.

In total, 441 first responders died in the attacks, the largest loss of emergency personnel in American history.

Image source, Getty Images

This is the first major anniversary of the attacks where a generation of people has grown up with no recollection of the events that day.

Four planes flying over the eastern US were seized simultaneously by small teams of suicide attackers. Thousands were murdered in a matter of minutes.

The planes were used as guided missiles to crash into landmark buildings in New York and Washington.

Two planes struck the Twin Towers of the World Trade Center in Manhattan and a third plane hit the Pentagon - the US military headquarters just outside Washington DC.

The fourth plane crashed in a field in Pennsylvania after passengers fought back against their hijackers.

In all, 2,977 people (not counting the 19 hijackers) lost their lives, most of them in New York.

Read more on the event and its consequences

The Queen has sent a message to US President Biden this morning.

She says her thoughts and prayers are with the victims, survivors and their families, as well as the first responders and rescue workers.

In her message, she also paid tribute to the resilience of the communities who joined together to rebuild.

"My visit to the site of the World Trade Center in 2010 is held fast in my memory," she says.

"It reminds me that as we honour those from many nations, faiths and backgrounds who lost their lives, we also pay tribute to the resilience and determination of the communities who joined together to rebuild."

Image source, Getty Images

Image source, Getty Images

Memorials and vigils are being held across the US with major events being held at the three sites where the attacks took place.

In New York

At the 9/11 Memorial Museum, which occupies the site of the destroyed World Trade Center’s Twin Towers, names of victims will be read out from 08:46 local time (13:46 BST). The reading will be paused six separate times to acknowledge key moments from the day.

In Washington DC

Secretary of Defense Lloyd Austin and Chairman of the Joint Chiefs of Staff Gen Mark Milley will host an observance ceremony at the Pentagon for the 184 people who died there.

The Democratic and Republican leaders of the houses of Congress will lead lawmakers in a moment of silence.

In Pennsylvania

An official ceremony will be held for the victims of Flight 93, which crashed in a field in Shanksville. Former President George Bush is expected to attend and deliver remarks.

President Joe Biden is also due to appear at all three events throughout the day.

Welcome to our live coverage of the memorial events for the September 11 attacks.

Four passenger planes hijacked by al-Qaeda killed nearly 3,000 people in the US.

We will bring you the latest from the events happening in three US cities, and explain how the attacks 20 years ago still have an impact on the world today.