John Walker Lindh said ISIS was 'doing a spectacular job' after beheading ...

John Walker Lindh, the American Taliban who is set to go free on Thursday after spending 17 years in prison, said in 2015 that ISIS was 'doing a spectacular job' after the Islamic group beheaded U.S. journalists.

Lindh expressed support for ISIS in a series of letters he wrote from prison to a producer at KNBC-TV in Los Angeles. 

Lindh, who is originally from Marin County, California, was asked about his reaction to ISIS beheadings of Americans, including journalist James Foley.

'We are in prison due to our beliefs and the practice of our religion, not for committing any crime,' Lindh wrote from the Federal Correctional Complex in Terre Haute, Indiana, in a March 2014 letter. 

On August 19, 2014, ISIS circulated a video online showing the moments before the beheading of Foley, who was in Syria covering the civil war for Agence France-Presse and GlobalPost. 

Lindh, the U.S. national who joined the Taliban and was captured by American forces during the invasion of Afghanistan in November 2001, was asked his response to the beheadings of Foley and others and whether ISIS represented Islam.

'Yes, and they are doing a spectacular job,' he wrote in February 2015. 

'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 letters to KNBC-TV have national security officials worried that after his release from prison, he will pose a threat.

In 2016, the National Counter Terrorism Center prepared a memo which found that Lindh 'continued to advocate for global jihad and to write and translate violent extremist texts.'

Government officials have read similar reports about Lindh in recent weeks, according to NBC News. 

Outrage is mounting on the eve of Lindh's release.

Foley's mother appeared on Fox News on Thursday and said she does not believe a man who supported her son's killers should walk free.

'I don't think he should be released if he is going to continue to sow hate and terrorism around the world,' Diane Foley said in an interview with on Fox & Friends First on Thursday morning. 

Foley went on to say that anyone accused of 'incredible human rights crimes' should be 'put away for the rest of their lives' if found guilty. 

She commented on the type of justice she'd like to see for those who commit acts of terror against Americans.

'I would like them brought back to the United States and tried in federal criminal court so all their crimes can be brought out in the open, and they can be, if convicted, they can be held in prison for the rest of their lives,' Foley said. 

'That's what I would like.'

Others expressed similar sentiments. 

'He shouldn't be on parole at all,' said Rep. Michael Waltz, a Florida Republican and former Green Beret who served in the War in Afghanistan, in an interview on Wednesday with Fox & Friends. 

'He should be in prison for life, he's a traitor,' Waltz said. 

'We're calling on the Federal Bureau of Prisons to explain why he is being released early, and then we are calling on law enforcement authorities to be watching him extremely closely,' Waltz said. 'We need to keep a close eye on him.'

Waltz is not the only lawmaker expressing outrage at the release of 38-year-old Lindh, who became known as 'Detainee 001' in the war on terror after 9/11.

In a letter this week to the Federal Bureau of Prisons, two senators asked how he would be contained, citing allegations that he continues to 'openly' support extremist violence.

'We must consider the security and safety implications for our citizens and communities who will receive individuals like John Walker Lindh,' they said.

Origins of the American Taliban fighter

The quiet son of a middle-class couple living north of San Francisco, Lindh converted to Islam at age 16 after seeing the film 'Malcolm X', and traveled to Yemen in 1998 to study Arabic and the Quran. 

In November 2000, he went to Pakistan and from there made his way to Afghanistan. 

He joined the Taliban and was with them on September 11, 2001, when al-Qaida terrorists attacked the World Trade Center and the Pentagon.

The U.S. attacked Afghanistan after the country failed to turn over al-Qaida leader Osama bin Laden. 

Lindh was captured in a battle with Northern Alliance fighters in late 2001. He was present when a group of Taliban prisoners launched an attack that killed Johnny Micheal 'Mike' Spann, a CIA officer who had been interrogating Lindh and other Taliban prisoners.

Television footage of a bearded, wounded Lindh captured among Taliban fighters created an international sensation, and he was brought to the U.S. to face charges of conspiring to kill Spann and providing support to terrorists. 

Eventually, he struck a plea bargain in which he admitted illegally providing support to the Taliban but denied a role in Spann's death.

Incarceration and early release 

Lindh is being released from the high-security prison in Terre Haute, Indiana three years early for good behavior in a 20 year sentence. 

He served roughly 17 years and five months, including two months when he was in military detention. Federal inmates who exhibit good behavior typically serve 85 per cent of their sentence.

Lindh has been housed in Terre Haute, Indiana, with other Muslim inmates convicted on terror-related charges. The rationale was to keep those inmates from radicalizing others in the general prison population. 

Those inside the unit were supposed to be limited in their ability to communicate with each other, but experts say the reality is they likely do talk. 

A former inmate who knew Lindh from the time they spent at the same federal prison said he never heard Lindh espouse support for al-Qaida or indicate a risk for violence, but he found Lindh to be anti-social and awkward around others, with an unyielding, black-and-white view of religion. 

The inmate spoke to The Associated Press on condition of anonymity because he wanted to avoid further stigmatization from his time in Lindh's prison unit. 

Alexander Meleagrou-Hitchens, a researcher at the George Washington University Center for Extremism, says that while in prison Lindh became close Ahmad Musa Jibril, an Arab-American who since his 2012 release continues to preach an extremely conservative version of Islam popular among jihadists. 

Lindh's time in prison has provided only a few clues about his current outlook. He filed multiple lawsuits, which were largely successful, challenging prison rules he found discriminatory against Muslims. In the more recent lawsuits, he used the name Yahya Lindh.