Salim Ahmed Hamdan provided a service to al-Qaida as bin Laden's driver and bodyguard from 1996-2001, including around the Sept. 11, 2001, terrorist attacks on the United States that al-Qaida members carried out, the U.S. Circuit Court of Appeals for the District of Columbia acknowledged.
But under the international law of war in effect at the time of his actions, no such defined war crime existed, Judge Brett Kavanaugh wrote for the court's 3-0 ruling, The New York Times reported Tuesday.
"Therefore the relevant statute at the time of Hamdan's conduct ... did not proscribe material support for terrorism as a war crime," Kavanaugh wrote.
Kavanaugh, who worked as a lawyer in the George W. Bush White House before Bush appointed him to the bench, was joined in his opinion by Chief Judge David Sentelle and Judge Douglas Ginsburg, appointees of President Ronald Reagan.
"If the government wanted to charge Hamdan with aiding and abetting terrorism or some other war crime that was sufficiently rooted in the international law of war at the time of Hamdan's conduct, it should have done so," Kavanaugh wrote.
Hamdan, captured at an Afghan roadblock in 2001, was held as an accused terrorist at the Guantanamo Bay, Cuba, detention camp until 2008.
The court also ruled U.S. military tribunals were not authorized to try Guantanamo prisoners suspected of providing material support to terrorist groups before 2006 because the Military Commissions Act of 2006 -- which explicitly makes providing material support for terrorism a crime -- does not authorize such retroactive prosecutions, the three-judge panel said.
The ruling could affect Guantanamo detainees who stand accused of being part of al-Qaida before 2006 but did not plot any specific terrorist act, The New York Times said.
American Civil Liberties Union senior staff attorney Zachary Katznelson told the newspaper the decision "strikes the biggest blow yet against the legitimacy of the Guantanamo military commissions, which have for years now been trying people for a supposed war crime that in fact is not a war crime at all."
Hamdan's case spurred Congress to create the Military Commissions Act after the U.S. Supreme Court ruled in Hamdan vs. Rumsfeld in 2006 that the military commission system set up by the Bush administration to prosecute Guantanamo detainees for war crimes lacked "the power to proceed because its structures and procedures violate both the Uniform Code of Military Justice and the four Geneva Conventions signed in 1949."
The congressional rewrite led to a 2008 military jury trial and 66-month conviction for Hamdan on eight counts of material support. He was acquitted of a conspiracy charge.
Because Hamdan had already served so long in prison, he was released later in 2008 to Yemen, his home country.
Even after his release, Hamdan continued to challenge his conviction.
The U.S. Court of Military Commission Review, created by the Military Commissions Act to appeal Guantanamo military commission rulings, upheld his conviction last year.