To mark the 18th anniversary of the 9/11 attacks, commemorative services are being held in the US and all over the world to honour the lives lost during the tragedy.
Almost two decades ago, the world watched as four terror attacks were committed against the US, including two hijacked planes crashing into the World Trade Center in New York.
Millions of people will commemorate 9/11 in their own ways, including attending memorial services and participating in volunteer projects.
Here's what you need to know about the memorial services in the US, including what time the minute's silence starts:
What time is the minute's silence held?
A moment of silence is observed to correspond with the attacks, beginning at 8:46am EDT, when the first plane struck the North Tower of the World Trade Center.
In the UK, the minute silence will be held at 1:46pm BST.
How will the US commemorate 9/11?
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Thousands of victims’ relatives, survivors and rescuers are expected at the anniversary ceremony at the World Trade Center in New York.
Normally opened for visitors daily, the 9/11 museum in New York City will be closed to the public for the entirety of Tuesday. The Memorial will be closed for the beginning of the day and open at 3pm EDT.
An additional memorial service will be held at the Pentagon, where one of the planes crashed. President Donald Trump will attend the ceremony, which is closed to the public.