Haunting audio has been released of the panicked communications of first responders, air traffic controllers, pilots, flight attendants and citizens as September 11, 2001 unfolded.
The audio was released by the US Transportation Security Administration on the 17th anniversary of the terrorist attacks that killed an estimated 2,977 people.
The audio includes flight crew advising ground control of stabbings on board, reports of aircraft flying "very erratically", and the first reports of planes hitting the Twin Towers.
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"Our number one [cabin crew member] has been stabbed and our five [cabin crew member] has been stabbed," a panicked woman can be heard saying.
It also features people making the final calls to their loved ones from onboard the hijacked planes, and inside the World Trade Centre.
Nineteen men hijacked four US commercial airplanes, crashing them in New York City, Washington DC, and Pennsylvania.
At the World Trade Center (WTC) in Lower Manhattan, 2,753 people were killed after two planes were intentionally crashed into the north and south towers.
At the Pentagon in Washington, a plane crashed into the building and killed 184 people.
Outside of Shanksville, Pennsylvania 40 people died when a plane crashed into a field instead of its unknown target. It is believed passengers and crews had attempted to regain control of the flight.