This article has been produced as part of Yahoo's 'On This Day' series
Exactly two decades ago, nearly 3,000 people were killed when terrorists targeted buildings in the US during the 9/11 attacks.
The crisp New York morning on 11 September, 2001, was dramatically altered to a scene of chaos, destruction and tragedy after two airplanes collided into the World Trade Center in Manhattan.
The Pentagon in Washington DC was also hit by a plane, while passengers and crew on United 93 overpowered their hijackers before they could hit an intended target in Washington DC, thought to be either the White House or the Capitol.
United 93 crashed into a field in Somerset County, Pennsylvania, before terrorists could carry out their attack – killing all 44 people on board.
Saturday marks the 20th anniversary of what was the deadliest terrorist atrocity in US history – but Khalid Sheikh Mohammed, the mastermind behind the attacks, has still not faced trial and may not do so for another 20 years.
‘A perfect storm of complication’
Mohammed was captured in Pakistan in March 2003 and has been detained at Guantanamo Bay detention camp since 2006.
In 2008 he was charged with war crimes and murder and his pre-trial was restarted this week – but his defence lawyer believes the trial completion may not happen for two decades.
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David Nevin, who has represented Mohammed since 2008, explained to BBC Radio 4’s Today programme: “You have here a perfect storm of complication coming from many directions but most of it driven by the torture and resulting classification of almost everything related to the torture – which massively complicates management of the case.”
Watch: Why 'mastermind' of Sept 11 attacks 'may not face justice for another 20 years'
Nevin said “the order of magnitude is something on the order of 20 years for complete resolution of the process”, adding: “If there are appeals, they will be long.”
A new judge also has to familiarise himself with some 35,000 pages of transcripts of previous hearings and thousands of motions.
Nevin has described it as the "largest criminal trial in the history of the United States".
‘A cover-up of torture’
While Mohammed’s pre-trial resumed on Tuesday some experts agree with Nevin that it could be many years before any outcome – if a trial takes place at all.
Madeline Morris, a Duke law professor, who heads the Guantanamo Defence Clinic, told NBC News: ”What we've been doing is to have, in essence, no trial at all. It's not necessarily a foregone conclusion that there will be a trial."
James Connell, a lawyer for Guantanamo detainee Ammar al Baluchi, added that “the cover-up of torture” is “the reason that we are all gathered at Guantanamo for the 42nd hearing in the 9/11 military commission on the 15th anniversary of the transfer of these men”.
Defence lawyers also say that it has taken years to access classified evidence against their clients, and that they still don’t have everything they need for the trial, NBC reported.
While Osama Bin Laden is the man most closely associated with 9/11, it was Mohammed who was named as the “principal architect”, according to the 9/11 Commission, which investigated the attacks.
It was Mohammed who came up with the plan to attack the US using planes, and took his idea to al-Qaeda for approval.
He was linked to a plot to blow up international airliners over the Pacific in the 1990s but an attempt to arrest him in Qatar failed.
After Mohammed’s name was linked to the 9/11 attacks, it took nearly two years for him to be tracked down and arrested in Pakistan, after which he was taken to a CIA “black site” where he was subjected to "enhanced interrogation techniques” – including being waterboarded at least 183 times, forced nudity and sleep deprivation.
Despite confessing to multiple plots during this period, a Senate report later found that much of the so-called intelligence had simply been made up by Mohammed.
Attempts to hold a trial in New York were abandoned following a public and political backlash, while procedural delays and the COVID pandemic have drawn out the start of a military trial at Guantanamo.
Any conviction is likely to face years of appeals due to the method of torture used to obtain information.
Watch: The West 'could face another 9/11' following Afghanistan withdrawal