Americans are set to mark the 19th anniversary of the September 11 attacks.
Al-Qaeda’s co-ordinated attacks on the United States resulted in 2,977 fatalities – the single-deadliest terror attack in history – causing international outrage and leading to then US President George W. Bush’s war on terror.
Ceremonies at the 9/11 Memorial Plaza, the Pentagon and Stonycreek, Pennsylvania – the site of the attacks – are set to take place on Friday.
What happened on September 11, 2001?
Nineteen years ago on the morning of September 11 2001, millions of Americans woke up to shocking images of New York City’s iconic Twin Towers ablaze.
By the end of the day it became apparent that the nation had been the target of a terror attack of unprecedented scale.
This is how events unfolded:
8.38am local time
Air traffic controllers are informed that American Airlines Flight 11, a flight bound for Los Angeles had been hijacked.
8.45am local time
American Airlines Flight 11 crashes into the north tower of the World Trade Center in New York City.
9.03am local time
Just after 9pm a second hijacked flight, United Airlines Flight 175, crashed into the south tower of the World Trade Center.
9.37am local time
The Pentagon in Washington DC is targeted by a third attack from the third hijacked plane, American Airlines Flight 77.
9.59am local time
The south tower of the World Trade Center collapses, killing and trapping those inside, including emergency workers.
10.03am local time
A fourth flight, United Airlines Flight 93 crashes into a field outside of Stonycreek Township, Pennsylvania. It has been suggested that the US Capitol was the target of the hijackers.
10.29am local time
The north tower of the World Trade Center collapses.
Who was responsible for the attacks?
Nineteen hijackers took over the four planes involved in the attacks.
They were all members of Islamist terrorist group Al-Qaeda, led by Saudi Arabian Osama Bin Laden, who founded the organisation in the late 1980s.
Bin Laden initially denied involvement in the attacks stating “I stress that I have not carried out this act” five days after the attacks.
In a video released in December that year, he praised the attacks.
He said: “It has become clear that the West in general and America in particular have an unspeakable hatred for Islam. ... It is the hatred of crusaders.
"Terrorism against America deserves to be praised because it was a response to injustice, aimed at forcing America to stop its support for Israel, which kills our people. ... We say that the end of the United States is imminent, whether Bin Laden or his followers are alive or dead, for the awakening of the Muslim umma (nation) has occurred.”
Shortly before the 2004 US election he admitted responsibility for the attacks in a video statement.
Osama Bin Laden was killed by US special forces in 2011.
What happened after 9/11?
In October the United States and United Kingdom invaded Afghanistan. At the time the government was led by the Taliban government who were apparently sympathetic to Al-Qaeda.
Native troops, supported by the United States, drove out most of the Taliban by the turn of the year, but conflict continues to plague the country with the Taliban carrying out attacks on a regular basis.
Tens of thousands of people died as a result of the invasion and the subsequent war.
In 2003 the US and UK led an invasion of Iraq, overthrowing dictator Saddam Hussein.
Both US and UK governments argued that Iraq was a threat to other countries and sympathetic to terror groups, including Al-Qaeda.
Though Hussein was quickly ousted, violence and war has plagued the region since with tens of thousands of casualties recorded.
How is 9/11 being remembered in 2020?
Due to the coronavirus pandemic, commemorations will look different in 2020 with social distancing still in place in the US.
In New York family members of attack victims will read out the names of loved ones in a pre-recorded video which will be streamed online.
Vice President Mike Pence will attend a ceremony at the September 11 memorial plaza.
Beams of light resembling the Twin Towers will light up New York’s skyline from dusk on Friday.
President Donald Trump and Democratic challenger Joe Biden will attend the Flight 93 National Memorial in Pennsylvania.
Every year since the attacks moments of silence have been observed to correspond with the attacks, beginning at 8:46am EDT – this takes place at 1.46pm UK time.