This year marks the 21st anniversary of the 9/11 attacks in New York. On 11 September 2001 almost 3,000 people were killed when terrorists hijacked four passenger planes and crashed them in to the twin towers at the World Trade Centre in New York, the Pentagon and a field in Pennsylvania.
The terrible events of that day shocked and saddened not only people in the United States, but also in the UK and around the world. Every year since, the sombre date has been marked with a series of memorial events.
But just what are the events taking part in 2022, and how can we watch them in the UK? Here is everything you need to know.
What is happening at Ground Zero?
Since 2002, each year on September 11, the names of the dead have been read at the event at the famous memorial pools, an event previously attended by Presidents Bush and Obama.
The service will play out similarly this year for the 21st anniversary of the terrorist attack.
Vice President Kamala Harris and her husband will go to New York City for the commemoration ceremony this year.
Commemorations will take place today (11 September) at the National September 11 Memorial & Museum in New York City, starting at 1.40pm UK time.
Following on from previous years, moments of silence will mark when a plane hit each of the Twin Towers, the Pentagon and a field in Shanksville, Pennsylvania, and when each tower fell.
The service is expected to conclude at 6pm UK time.
Where else are there memorials?
A memorial will also be held at the Flight 93 National Memorial in Pennsylvania. A field in Pennsylvania is where the fourth plane crashed after it had been hijacked by terrorists.
Jill Biden, the first lady, will also speak at the memorial on Sunday 11 September. The "September 11 Observance" is "free and open to the public," according to the National Park Service (NPS) website.
How will President Joe Biden mark the anniversary?
The White House says President Joe Biden will mark the 21st anniversary of the 9/11 attacks on Sunday 11 September by delivering remarks and laying a wreath at the Pentagon. The Pentagon was the location that the third plane crashed after it had been hijacked by terrorists.
How can I watch the memorials in the US and the UK?
The 9/11 memorial services in New York City and Pennsylvania will be broadcast live online by several major television networks.
NBC News, CNBC, ABC News, CBS News, Fox Business, and many others have previously live streamed the 9/11 commemorations on their YouTube channels.
The events are also expected to be broadcast through the "Watch Live" section of the 9/11 Memorial & Museum website, where the museum has live streamed other events, so that people in the UK can also watch.
Will there be a minute’s silence in the UK?
While there is no “official” minute’s silence, many people choose to observe a moment of quiet contemplation at 1.46pm UK time, the moment the first plane struck the Twin Towers.