John Walker Lindh, the American captured in Afghanistan in 2001 fighting for the Taliban, has been released early from a US federal prison, the Washington Post reports, citing Lindh's lawyer.
Lindh, who was 20 years old when he was captured, left prison in Terre Haute, Indiana, on probation after serving 17 years of a 20-year sentence, the newspaper said.
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Now 38, Lindh is among dozens of prisoners set to be released over the next few years after being captured in Iraq and Afghanistan by US forces and convicted of terrorism-related crimes following the attacks by al Qaeda on September 11, 2001.
His release brought objections from elected officials who asked why Lindh was being freed early and what training parole officers had to spot radicalisation and recidivism among former jihadists.
US Secretary of State Mike Pompeo called Lindh's release "unexplainable and unconscionable".
"There's something deeply troubling and wrong about it," he said on Fox News on Thursday morning.
Leaked US government documents published by Foreign Policy magazine show the federal government as recently as 2016 described Lindh as holding "extremist views".
US-born Lindh converted from Catholicism to Islam as a teenager. At his 2002 sentencing he said he travelled to Yemen to learn Arabic and then to Pakistan to study Islam.
He said he volunteered as a soldier with the Taliban, the radical Sunni Muslim group that ruled most of Afghanistan from 1996 to 2001, to help fellow Muslims in their struggle or "jihad".
He said he had no intention "to fight against America" and never understood jihad to mean anti-Americanism.
Lindh told the court he condemned "terrorism on every level" and attacks by al Qaeda leader Osama bin Laden were "completely against Islam".
But a January 2017 report by the US government's National Counterterrorism Center, published by Foreign Policy, said that as of May 2016, Lindh "continued to advocate for global jihad and to write and translate violent extremist texts".
NBC News reported that Lindh wrote a letter to its Los Angeles station KNBC in 2015 expressing support for Islamic State, saying the Islamic militant group was fulfilling "a religious obligation to establish a caliphate through armed struggle".