Khalid Sheikh Mohammed, the supposed mastermind behind 9/11, is still awaiting trial over two decades on from the events that killed 2,974 people.
Having first been arrested in March 2003, 18 months on from the September 2001 attack, plans to try Mohammed in a military or even civilian court have been delayed several times.
This means that the Pakistani-born suspect has spent almost 20 years at Guantanamo Bay - despite never actually standing trial.
Mohammed was first identified as as a high-profile terror suspect almost immediately after 9/11, and it was in Rawalpindi, Pakistan, that he was eventually captured by US forces.
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According to some experts, the difficulty authorities face in trying the 58-year-old in a civilian court comes from the fact that he was subjected to 'enhanced interrogation techniques' by CIA operatives.
In the years following 9/11, critics have labelled these 'techniques' as being tantamount to torture, which would be in violation of the Geneva Convention.
Interrogation methods supposedly included waterboarding and electrocutions, which would immediately discredit anything Mohammed confessed to under the US legal system.
David Kelley, a former US Attorney General and strong critic of the treatment of alleged terrorists at Guantanamo, regards their lack of a fair trial as an 'awful tragedy' for the families of the victims.
Having co-chaired the Justice Department’s nationwide investigation into the attacks, Kelley said the situation at Guantanamo was 'a tremendous blemish on the country’s history'.
Amnesty International report that of the 107 detainees at Guantanamo in 2020, 47 had been cleared for transfer - only to remain without a reason being specified.
In the case of Mohammed, there is little to no hope of a transfer, and attempts to get a trial underway continue to face setbacks.
Pretrial hearings for Mohammed and four other alleged 9/11 conspirators were set to begin later this month, but ended up being cancelled.
With the exact details over his alleged involvement in the 9/11 attacks still unclear, some families of the victims claim that they just want closure.
George Haberman's 25-year-old daughter lost her life in the attacks after one of the planes hit the floor above her office. He says it's crucial that Mohammed faces justice.
He said: “It’s important to me that America finally gets to the truth about what happened, how it was done
“I personally want to see this go to trial."
According to CBS News, the attorneys for Mohammed, as well as four other defendants, could try and negotiate a plea deal that would take the death penalty off the table when the trial does eventually get underway.
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Tom Fenton