When remembering the horrific events of 9/11 in which almost 3,000 people died, most people think of the collapse of the Twin Towers, which was broadcast live on TV for many of us.
However as we approach the 21st anniversary of the September 11 attacks, we should also remember the tragedies that took place following the hijacking of two more planes.
al-Qaeda terrorists hijacked a third passenger jet, which flew into the west side of The Pentagon building, and a fourth, United Airlines Flight 93, which crashed in a field in Pennsylvania.
Here is everything you need to know about each of the four planes involved.
American Airlines Flight 11
American Airlines Flight 11 was a domestic passenger flight travelling from Boston to Los Angeles when it was hijacked by five al-Qaeda terrorists.
Fifteen minutes into the flight, the hijackers injured at least three people (possibly killing one), and forcibly breached the cockpit to overpower the captain and first officer.
Lead hijacker Mohamed Atta was a licensed commercial pilot and took over the controls.
He deliberately crashed the plane into the North Tower of the World Trade Centre at 8:46am local time.
Many people in the streets of New York City witnessed the strike, but very few video recordings captured the moment; documentary filmmaker Jules Naudet captured the only known footage of the initial impact from start to finish.
The impact and subsequent fire caused the North Tower to collapse 102 minutes after the crash, resulting in hundreds of additional fatalities.
United Airlines Flight 175
United Airlines Flight 175 was also scheduled to fly between Boston and LA, but 30 minutes into the flight, hijackers forcibly breached the cockpit.
Hijacker and trained pilot Marwan al-Shehhi took over the controls, and crashed the aircraft into the South Tower of the World Trade Center at 9.03am, 17 minutes after the impact of American Airlines Flight 11.
The South Tower collapsed 56 minutes after it was hit by the aircraft.
American Airlines Flight 77
American Airlines Flight 77 was scheduled to depart the Washington Dulles International Airport in Virginia, on its way to Los Angeles International Airport.
But less than 35 minutes into the flight - shortly after American Airlines Flight 11 struck the North Tower of the World Trade Centre - the Boeing 757-223 aircraft was hijacked by five men affiliated with al-Qaeda, who stormed the cockpit and forced the passengers, crew, and pilots to the rear of the aircraft.
Hijacker Hani Hanjour - who had trained as a commercial pilot at the CRM Airline Training Center in Arizona in 1999 - assumed control of the flight.
Normal radio communications to air traffic control ceased, and a few minutes later the plane began to deviate from its normal assigned flight path, turning south.
The plane’s transponder was then switched off as the hijackers set the flight’s autopilot on a course heading east towards Washington, DC.
Air traffic controllers asked a passing fighter jet to follow the aircraft, but the pilot said he had difficulty picking out the airplane in the “East Coast haze”. He then saw a “huge” fireball and assumed the aircraft had hit the ground.
An hour and 17 minutes after takeoff - at 9.37am - the plane was deliberately crashed into the western side of the Pentagon, the impressive five-sided structure that serves as the headquarters for America’s military.
While level above the ground and seconds from impact, the Boeing’s wings clipped five street lamp posts, before impacting the Pentagon at the first-floor level at a speed of 530mph.
The impact severely damaged an area of the Pentagon and caused a large fire. A portion of the building later collapsed, and firefighters spent days working to fully extinguish the blaze.
All 64 aboard the aircraft - including six crew and the five hijackers - died, as well as another 125 personnel inside the building, taking the death toll to 189.
United Airlines Flight 93
United Airlines Flight 93 was a domestic scheduled passenger flight flying from Newark International Airport in New Jersey to San Francisco International Airport in California when it was hijacked by four al-Qaeda terrorists.
The hijackers stormed the aircraft’s cockpit just 46 minutes after takeoff on September 11 2001, and a struggle - transmitted to Air Traffic Control - ensued between the captain, Jason Dahl and first officer LeRoy Homer Jr.
Ziad Jarrah, a member of Al-Qaeda who had trained as a pilot, took control of the aircraft. He told passengers: “Keep remaining sitting. We have a bomb on board.”
The passengers were moved to the back of the plane and Jarrah diverted it back towards the east coast, in the direction of Washington DC.
Many of the passengers and crew had already heard through phone calls about the attacks on the World Trade Center and The Pentagon. But rather than giving up, many of them tried to take back control of the plane.
After the plane’s voice data recorder was recovered, it revealed pounding and crashing sounds against the cockpit door and shouts and screams.
One passenger cried “Let’s get them!” and a hijacker shouted “Allahu akbar”. Jarrah tried to knock passengers off their feet by rocking the plane but the passengers continued their efforts.
A male passenger is heard in the recording saying, “Turn it up!” and a hijacker says, “Pull it down! Pull it down!”
At 10.03am on September 11 2001, Flight 93 crashed into a field near Indian Lake and Shanksville, Pennsylvania.
The flight hit the ground upside-down at a 40-degree nose-down angle. It was flying at 563mph on impact, and left a crater eight to 10 feet deep and 30 to 50 feet wide.
Flight 93’s intended target has never been conclusively confirmed, but it’s been speculated that it was either the White House or The Capitol building.
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