John Walker Lindh, the so-called "American Taliban" whose capture in Afghanistan riveted a country in the early days after the September 11 attacks, has been released from prison.

After serving 17 years of a 20-year sentence, Lindh, the first US-born detainee in the war on terror, on Thursday walked out of a federal prison in Terre Haute, Indiana, and will join the small, but growing, group of Americans convicted of terror-related charges attempting to re-enter into society. 

Lindh will live in Virginia subject to the direction of his probation officer, his lawyer, Bill Cummings, tells CNN. But some are already calling for an investigation into his time in prison -- where he is said in two US government reports to have made pro-ISIS and other extremist statements -- that could send him back into detention. 

Reports of Lindh's maintained radicalization, detailed in two 2017 official counterterrorism assessments, are also driving questions about the efforts of the US government to rehabilitate former sympathizers like him, who are expected to complete prison sentences in waves in the coming years. 

Raised in the suburbs north of San Francisco, Lindh took an interest in Islam at a young age, converting to the religion at 16 and moving to the Middle East to learn Arabic after finishing high school. 

In 2000, according to documentation of his interrogations, Lindh went to Pakistan and trained with a radical Islamic group there before moving to Afghanistan and joining the Taliban. 

At a sentencing hearing in Virginia that year, he sniffled and nearly broke down as he addressed the court in a 14-minute speech. 

"Had I realized then what I know now about the Taliban, I would never have joined them," Lindh said. "I never understood jihad to mean anti-Americanism or terrorism." 

That contrition has been contested by a pair of official reports, from the National Counterterrorism Center and the federal Bureau of Prisons, that were first published by Foreign Policy in 2017. 

Read more: CNN