When Americans honour the nearly 3 000 killed in the 11 September 2001 al-Qaeda attacks on the 20th anniversary on Saturday, a grim shadow will lurk in the background, that of attacks mastermind Khalid Sheikh Mohammed, who has yet to be tried and convicted for the heinous crime.
Mohammed, who boasted to interrogators of designing and managing the 9/11 plot, sits in a cell in a high-security camp on the US naval base at Guantanamo Bay, Cuba.
He has been here for 15 years, as the attempt to hold him accountable in a US military war court plods on in fits and starts, stuck on whether his being tortured by the CIA renders his boastful confessions inadmissible.
He remains, after the now-dead Osama bin Laden, the most reviled figure tied to the 9/11 attacks.
Appearing in the Guantanamo military courtroom for the first time in over 18 months this week, Mohammed walked in easily, chatting with a fellow defendant in the death-penalty case, and knelt on a small carpet between the tables for prayers.
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The official 9/11 Commission report and a Senate report about the CIA’s torture programme describe the 56-year-old “KSM” as a capable lieutenant of Bin Laden.
A Pakistani citizen, he was raised in Kuwait. He learned English well enough to study mechanical engineering at a US university.
Following his transfer to Guantanamo in September 2006, he confessed to the military court and compared himself to George Washington, fighting to escape oppression.
“I was responsible for the 9/11 operation, from A to Z,” he said.