Internet entrepreneur Danny Lewin, 31, was travelling on American Airlines Flight 11 when it was hijacked just 16 minutes into a journey from Boston to Los Angeles as it passed over Worcester, Massachusetts.
The hijackers then turned the plane north instead of south - heading towards New York airspace.
A veteran of an elite unit in the Israeli Defence Force who understood Arabic, Lewin appears to have immediately understood what was happening and challenged the terrorists, who were armed with box-cutters and knives.
It was an act of extraordinary courage which was to cost him his life. As the former commando fought with Satam al-Suqami, one of the five who had taken the plane, he was fatally stabbed.
Half an hour later, at 8.46am, the Boeing 767 became the first plane to crash into the Twin Towers. All 81 passengers, nine crew members and the two pilots died.
The story of Lewin's unimaginable bravery has only just been made public in a new biography No Better Time: The Brief, Remarkable Life of Danny Lewin, the Genius Who Transformed the Interne.
Author Molly Knight said she wanted to write about Lewin's life because she felt it deserved to be celebrated.
She told CNN: 'It was about the way he lived. I felt like if he was motivating me as much, I felt like I could do the same for readers.'
The writer was able to piece together his last moments from the desperate accounts of two flight attendants who had been in contact with the ground as the tragedy unfolded.
Lewin's best friend Marco Greenberg told Slate: 'He was the first victim of the first war of the 21st century.'
There were to be many more. Seventeen minutes after Flight 11 crashed, United Airlines Flight 175 hit the South Tower of the World Trade Centre at 9.03am.
Then came the news that a third hijacked plane had crashed into the Pentagon in Arlington, Virginia, before a final aircraft was brought down in Shanksville, Pennsylvania.
The second hit: The moment Hijacked United Airlines Flight 175, which departed from Boston en route for Los Angeles, crashes into the South Tower of the World Trade Centre
Devastation: An explosion rips through the South Tower of the World Trade Centre after the hijacked United Airlines Flight 175 crashed into it. The North Tower is shown burning after American Airlines Flight 11 crashed a little over 15 minutes earlier
As news of terrorist attacks spread around the world, it was Lewin's own technology which allowed the internet to keep up with the increased amount of traffic.
The Israeli-American co-founded Akamai Technologies which is responsible for more than 30 per cent of the world’s internet traffic.
His company's October 1999 share market float made Lewin an overnight billionaire and he became one of the world's richest high-tech entrepreneurs before he was even 30-years-old.
The multibillion dollar company's clients today still include Sony, Apple and News Corp.
But before his business success, Lewin, who was brought up in Denver, Colorado, before moving to Israel with his family when he was 14, served with one of the country’s most elite counter-terrorism unit, Sayeret Mat’kal.
Killers: Waleed M Alshehri, pictured left, and Wail Alshehri, pictured right, were two of the hijackers on Flight 11
Terrorists: Abdulaziz Alomari, pictured left, and Mohamed Atta, pictured right, also helped to take over the plane
9/11: Nearly 3,000 people died when four hijacked planes were used in coordinated strikes on the Pentagon and the World Trade Center towers (pictured). The fourth plane crashed in Pennsylvania
He studied at the Israeli Institute of Technology before going on to Massachusetts Institute of Technology (MIT) on a full scholarship where he came up with a ground-breaking idea to allow the internet to work more efficiently and at faster speeds.
The set of algorithms he created and called 'consistent hashing' formed the basis of his company Akamai which he set up with MIT professor Tom Leighton in 1998.
Describing Lewin, Leighton told CNN: 'He was really exceptionally smart. MIT has a lot of really smart people, and Danny stood out even among that rarified environment.
In memory: A plaque for Danny Lewin on the fence of a park named in his memory in Cambridge, Massachusetts, where he studied at MIT
Gone but not forgotten: After Lewin's death, the intersection of Main and Vassar Streets in Cambridge, Massachusetts, was renamed Danny Lewin Square in his honor
Success story: Danny Lewin created a set of ground-breaking algorithms which make the internet work more efficiently and formed the basis of his company Akamai which he co-founded with MIT professor Tom Leighton in 1998
'He liked working on the hardest problems, as opposed to the easier ones, because they would make more of a difference.'
He added he was intrigued by Lewin's idea because it would make a real beneficial impact on the world.
He said: 'In the area where we worked, in algorithms and the theoretical side of computer science, often that work is good, deep work, but it doesn't change the world. It doesn't impact people directly.
'With this work, we thought it would have relevance in the real world, and make the Internet be faster, more reliable, more secure. It was a chance that was pretty rare for us.'
Memorial: President Barack Obama, First Lady Michelle Obama, Vice President Joe Biden and Jill Biden observe a moment of silence to mark the 12th anniversary of the 9/11 attacks on the South Lawn of the White House in Washington
Memorial: President Obama, the First Lady, Vice President Joe Biden and his wife Jill remember the moment of the attacks
Emotional: President Barack Obama, accompanied by Defense Secretary Chuck Hagel, wipes his face as he speaks during a 9/11 remembrance ceremony at the Pentagon on Wednesday morning
Akamai is responsible for keeping some of the world's most popular websites running smoothly, including Facebook and iTunes.
Lewin had boarded Flight 11, which took off shortly before 8am on September 11, to attend a Akamai business meeting in Los Angeles.
After the terrorist attacks, almost every major news site remained up and running that day despite the hug volume of traffic because of Lewin's creation.
If he was still alive today, it is likely he would be placed in the ranks alongside Steve Jobs and Bill Gates.
Lewin is survived by his wife Anne and sons Eitan and Itamar.
US President Barack Obama, left-rear, and others salute during the playing of the US National Anthem during a memorial service at the National 9/11 Pentagon Memorial
Paying their respects: The World Trade Center Flag is presented as friends and relatives of the victims of the 9/11 terrorist attacks gather at the National September 11 Memorial
Tributes: Flowers and pictures are displayed by a name along the north reflecting pool at the 9/11 Memorial to mark the anniversary of the attacks
