The hearing was held at a Guantánamo Bay military tribunal in Cuba.
The defendants included Khalid Sheikh Mohammed, who was accused of orchestrating the plot, and four other accomplices: Ammar al-Baluchi, Walid bin Attash, Ramzi bin al-Shibh and Mustafa al-Hawsawi.
Khalid Sheikh Mohammed and his four co-accused had been detained at a prison under the US military prison at Guantanamo Bay for nearly 15 years. Their latest hearing was held in the beginning of 2019.
Khalid Sheikh Mohammad and four others can face the death penalty if they are charged with murder, terrorism and war crimes charges.
The trial took place just days before the US preparing to mark the 20th anniversary of a series of terrorist attacks that shocked the world.
On September 11, 2001, the whole world was shaken as 19 militants associated with the Islamic extremist group al-Qaeda hijacked four airplanes, then turned them in turn and attacked a series of targets.
Two of them crashed into the North and South towers of the World Trade Centre (WTC) complex in New York City. The third one, American Airlines flight 77, crashed into the Pentagon in Arlington County, Virginia, partially collapsing the west side of the building.
Meanwhile, the United Airlines flight 93 was initially aimed at Washington, DC, but crashed outside Shanksville, Pennsylvania after heroic passengers tried to control the hijackers.
The series of attacks killed 2,976 people and injured more than 6,000 others as well as caused at least US$10 billion in property and infrastructure damage, leading to an estimated total loss of up to US$3 trillion.
They were also the deadliest incident ever for firefighting and law enforcement forces in the US history, with 343 firefighters and 72 law enforcement officers killed. The tragedy of September 11, 2001 has also left countless health consequences while causing persistent psychological wounds, especially for New Yorkers.
