What to Know
John Walker Lindh is set to be released Thursday after serving 17 years of a 20-year sentence
The Northern California man became known as the "American Taliban" after his capture by U.S. forces in the 2001 invasion of Afghanistan
During his time behind bars in Indiana, he responded with hand-written letters to questions from an NBC Los Angeles producer
Prison letters from the Northern California man who became known as the "American Taliban" after his capture by U.S. forces in the 2001 invasion of Afghanistan underscore concerns among authorities that he remains a potentially violent extremist, current and former officials told NBC News.
John Walker Lindh was released Thursday after serving 17 years of a 20-year sentence. He is being released early due to good behavior, standard in the federal prison system.
During his time behind bars in Indiana, he responded with hand-written letters to questions from an NBC Los Angeles producer. All correspondence in and out of the prison is subject to inspection.
In the letters, some with introductions on lined paper, Lindh identified himself as Yahya Lindh and said he spends his days in pursuit of Islamic knowledge. He called himself a political prisoner.
"We are in prison due to our beliefs and the practice of our religion, not for committing any crime," he wrote in a March 2014 letter.
He was asked whether the militant group ISIS represented Islam.
"Yes, and they are doing a spectacular job," he wrote in February 2015, going on to say he supports their efforts. "The Islamic State is clearly very sincere and serious about fulfilling the long-neglected religious obligation of establishing a caliphate through armed struggle, which is the only correct method."
Lindh's response was after ISIS beheaded Americans in well-publicized videos, including journalist James Foley in August 2014. Lindh did not respond to follow-up letters about the violence.
His correspondence with journalists and other comments in prison were part of the basis for a 2016 U.S. intelligence document from the National Counter Terrorism Center that said Lindh continued to "advocate for global jihad."
Lindh, who grew up in Marin County in the Bay Area, converted to Islam as a teenager after seeing the movie "Malcolm X." He then studied Arabic and the Quran overseas. In November 2000, he went to Pakistan and Afghanistan, where he joined the Taliban, according to his father.
He was with the group on Sept. 11, 2001, when terrorists conducted a series of coordinated attacks on the World Trade Center and the Pentagon.
When the United States attacked Afghanistan for its failure to turn over al-Qaida leader Osama bin Laden, Lindh was captured during a battle with Northern Alliance fighters. Television images showed Lindh, appearing dazed and exhausted among captured Taliban fighters.
The government would ultimately drop nine of 10 charges against him, including conspiracy to murder U.S. nationals. He pleaded guilty to supply services to the Taliban and a weapons charge.
When asked whether time behind bars changed his views, Lindh responded in a March 2014 letter: "I feel honoured to have been able to take part in the Afghan Jihad and to contribute to the defence of the Islamic Emirate of Afghanistan, albeit only in a very limited capacity."
During his sentencing, Lindh condemned terrorism and said it is never justified. He also said the Sept. 11 attacks were "against Islam." He was present for, but denied any role in an attack by Taliban prisoners that killed CIA officer Johnny Micheal "Mike" Spann, who interrogated Lindh.
Spann's father has protested the early release. The legislature in Alabama, where Spann grew up, approved a resolution calling Lindh's release an insult to Spann's legacy and family.
Lindh will be subject to mandatory monitoring of his internet use. He is banned from foreign travel and must undergo mental health counseling. A U.S. official told NBC News he would live in northern Virginia, which his lawyer confirmed to NBC Bay Area.
"It is one of the most restrictive sets of conditions I’ve seen in a terrorism case, and it probably speaks to their concerns about him," said Seamus Hughes, a former U.S. intelligence official who studies extremism at George Washington University.
Lindh did express remorse during his 2002 sentencing and said he did not support terrorism. However, a U.S. official told NBC News that a memo circulating this week described Lindh as a jihadi.
Lindh's lawyer and a representative of his family declined to comment to NBC News.
NBC News' Ken Dilanian contributed to this report.