On the morning of September 11, 2001, a total of 19 terrorists affiliated with al-Qaeda hijacked four commercial aeroplanes bound for California shortly after take-off from airports in Boston, New Jersey, and the Washington DC area.
Two would fly into each of the Twin Towers in lower Manhattan, New York City, while another crashed into the Pentagon and the fourth ended up in a field.
There were no survivors from any of the planes.
The attacks killed 2,977 people in the single largest loss of life from a foreign attack on American soil.
Among the dead were of 441 emergency services staff - no other day in American history has seen more first responders die.
September 11 had a drastic impact on US society and its foreign and domestic policy.
This is how the infamous day unfolded 20 years ago.
5.45am - Terrorists pass security
Cameras capture the moment hijackers Mohamed Atta and Abdul Aziz al-Omari go through security at Portland International Jetport in Maine.
Atta and al-Omari get on a flight to Boston Logan International Airport, which is where they connect to American Airlines Flight 11.
Three other hijackers will join Atta and al-Omari aboard this flight.
Before boarding Flight 77, Nawaf al-Hazmi, Khalid al-Mihdhar, and Majed Moqed set off metal detectors at Washington Dulles International Airport’s west checkpoint less than two hours later.
At the time, knives were allowed on planes as long as the blade was less than four inches long.
6am - Election day
September 11 was an election day in New York City and at 6am polls were opened for several offices, including the mayor’s.
7.59am - Flight 11 takes off
American Airlines Flight 11 departs from Boston and is supposed to fly to Los Angeles. On board are eleven crew members, 76 passengers, and the five hijackers.
8.15am - Flight 175 takes off
United Airlines Flight 175 also takes off from Boston and is meant for Los Angeles.
Nine crew members, 51 passengers, and five hijackers are on board.
8.19am - Flight 11 is hijacked
Barely 20 minutes after take-off, flight attendant Betty Ann Ong alerts American Airlines ground personnel to a hijacking underway on Flight 11.
This call lasts 25 minutes.
Shortly before Ong’s call, a hijacker, believed to be Satam al-Suqami, had stabbed the passenger seated directly in front of him in first class, who was later identified as Daniel Lewin by the flight crew and had served four years in the Israeli army.
Lewin could well have been the first victim of 9/11.
8.20am - Flight 77 takes off
American Airlines Flight 77, en route to Los Angeles, takes off from Washington Dulles International Airport.
Five hijackers are on board with six crew and 53 passengers.
8.24am - Hijacker accidentally alerts air traffic control
Hijacker Mohamed Atta presses the wrong button when trying to communicate with passengers on Flight 11, reaching out instead to air traffic control and alerting controllers to the attacks.
Atta did this more than once and at least one of his transmissions was picked up by the pilot of Flight 175, Victor Saracini, who would inform the Federal Aviation Administration of what he has heard minutes before his own plane is hijacked.
8.30am - Work begins at the World Trade Center
Morning activities began at the World Trade Center, which included four other office buildings and a hotel in addition to the Twin Towers.
Roughly 80 people gathered to attend the Risk Waters Group financial technology conference on the 106th floor of the North Tower.
8.37am - Military alerted
Boston air traffic control centre alerts the military
8.42am - Flight 93 takes off
Meant to leave Newark International Airport in New Jersey within minutes of the other hijacked flights, San Francisco-bound United Airlines Flight 93 takes off after a delay caused by routine traffic.
8.46am - Crash into North Tower
American Airlines Flight 11 crashes into floors 93 through 99 of the North Tower. All on board are killed instantly as well as hundreds in the building. This crash destroys all three emergency stairwells and traps hundreds of people above the 91st floor.
8.46am - Emergency services sent
New York City emergency dispatchers send police, paramedics, and firefighters to the North Tower.
8.50am - President George W Bush notified
During his visit to an elementary school in Florida, the president is alerted.
8.55am - South Tower declared safe
The South Tower was declared safe. According to 9/11 Memorial’s website, the announcement, made by Port Authority, said: “Your attention, please, ladies and gentlemen.
Building Two is secure. There is no need to evacuate Building Two. If you are in the midst of evacuation, you may use the reentry doors and the elevators to return to your office. Repeat, Building Two is secure.”
8.59am - Evacuation ordered
But four minutes later, the decision to evacuate both towers was made.
9.02am - Evacuation broadcast
The evacuation order is broadcast in the south tower
9.03am - Flight 175 hits the South Tower
United Airlines Flight 175 flies into floors 77 through 85 of the South Tower, killing all on board and an unknown number in the building.
Two of the three emergency stairwells are severed along with a majority of the elevator cables, trapping many above the impact zone.
Some workers in the building jump or fall to their deaths, something already witnessed after the attack on the North Tower.
According to 9/11 Memorial, It is estimated the number of people who die as a result of falling from the Twin Towers range from 50 to more than 200.
9.05am - The president is told again
George W Bush learns of the second attack.
9.12am - Phone call from Flight 77
From Flight 77, a flight attendant calls her mum to tell her hijackers have control of the plane.
9.36am - Dick Cheney evacuated
US Secret Service agents evacuate Vice President Dick Cheney to the Presidential Emergency Operations Center beneath the White House.
9.37am - Crash into Pentagon
Flight 77 crashes into the Pentagon. A total of 59 are killed on board, while 125 military and civilian personnel on the ground also die.
9.42am - Grounded
All flights are grounded
9.45am - Evacuations at White House
Evacuations take place at the White House and Capitol building.
9.53am - One of the last calls
From hijacked Flight 93, 37 telephone calls are known to have been made. One of the last calls is made by passenger Edward Felt, who dials emergency services number 911 after hiding in a toilet.
By 9:58 am, Flight 93 is flying so low that he succeeds in reaching an operator in Pennsylvania.
9.59am - South Tower collapses
After burning for 56 mins, the South Tower collapses in 10 seconds. More than 800 people are killed.
10.03am - Passenger revolt
Flight 93 crashes in a field near the town of Shanksville in Pennsylvania after passengers and crew stormed the cockpit.
The 33 passengers and seven crew members on board die.
10.15am - Part of Pentagon collapses
The Pentagon’s 'E Ring' offices collapse.
10.28am - North Tower collapses
The North Tower collapses after burning for 102 mins, killing more than 1,600 people.
11.02am - Evacuations
Lower Manhattan is evacuated.
12.16am - Air space shut
US air space is closed as the last flights in the air land.
A stairwell in the lower part of the North Tower’s survives the collapse, which sheltered a group of 14 people trying to escape.
Within hours of the tower’s collapse, they emerge from the debris.
5.20pm - Another building collapses
Collapse of 7 world trade center, burning for hours, the 47-storey tower had been evacuated earlier, no casualties
8.30pm - Address to the nation
The president addresses nation from white house.