A three-judge panel of the US Court of Appeals for the District of Columbia Circuit ruled 2-1 in favour of Ali Hamza al Bahlul, who made videos for Osama bin Laden's al Qaeda organisation.
Writing on behalf of the court, Judge Judith Rogers said the conviction had to be put aside because the special military tribunal, or commission, at Guantanamo did not have authority to convict Bahlul of a conspiracy charge. Conspiracy is not a crime recognised under the international law of war, Rogers said.
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The United States has used military commissions, first created during the administration of former President George W Bush, to put on trial a number of foreign terrorism suspects held at Guantanamo for alleged war crimes rather than prosecuting them in regular military courts or civilian courts.
Bahlul recorded recruiting videos and taped the wills of some of the hijackers who slammed commercial jetliners into New York's World Trade Center, the Pentagon outside Washington and a Pennsylvania field on September 11, 2001.
Three months after those attacks, Bahlul was captured in Pakistan and transferred to the Guantanamo Bay facility. A military commission convicted him of three crimes and sentenced him to life in prison at the detention center there.
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In July 2014, the appeals court threw out Bahlul's other convictions for providing material support for terrorism and solicitation of others to commit war crimes.
The case is Bahlul v United States, US Court of Appeals for the District of Columbia Circuit, No. 11-1324.
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