John Walker Lindh, the “American Taliban” fighter set to be released from prison on Thursday, praised ISIS for “doing a spectacular job," newly revealed prison correspondence shows.

The four letters, which Lindh wrote from prison to NBC’s Los Angeles station KNBC in 2014 and 2015, paint a picture of an unrepentant and violent radical.

“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 said in a 2015 letter.

Lindh, then 20, grew up in California and traveled to Afghanistan in early 2001 to fight alongside the Taliban and spent weeks at al Qaeda training camps, where he personally met and spoke with Osama bin Laden. He stayed with al Qaeda and the Taliban following the terrorist attacks of 9/11, and was captured by Northern Alliance fighters in November 2001.

Lindh was taken to Qala-i-Jangi fortress, where an uprising by the Taliban and al Qaeda prisoners killed CIA officer Mike Spann. Lindh was brought to the U.S. where he stood trial in 2002 and was sentenced to 20 years in prison. He is getting out on Thursday after less than 18 years behind bars.

During his sentencing in October 2002, Lindh told the court that he made a mistake by joining the Taliban and claimed “had I realized then what I know now about the Taliban I would never have joined them.” But years later, Lindh wasn’t remorseful.

In March 2014 Lindh wrote, “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.”

These prison letters are mentioned in at least one of two government assessments of Lindh, which provided evidence that Lindh looked favorably on radical Islamic terrorism from behind bars.

An intelligence summary from the Federal Bureau of Prisons quotes Lindh’s father Frank as saying that, in order to represent him again, Lindh’s former attorney “will absolutely demand that you be willing to condemn in all sincerity, publicly if needed, and without any reservation whatsoever, depravity of any kind, whoever commits it.”

But Lindh refused. “It looks like we will have to abandon this project because I am not interested in renouncing my beliefs or issuing condemnations,” he said.

An excerpt from a National Counterterrorism Center report stated that Lindh was actively engaging in extremist activity while in prison. “As of May 2016, John Walker Lindh … continued to advocate for global jihad and to write and translate violent extremist texts. In March 2016, he told a television news producer that he would continue to spread violent extremist Islam upon his release.”

Johnny Spann, the father of slain CIA officer Mike Spann, previously told the Washington Examiner that Lindh still poses a threat. “This guy has not changed and he's no model prisoner. He’s still a radical Islamic terrorist. If anything, he is more radicalized than ever.”

And Alison Spann, Mike Spann’s daughter, told the Examiner that Lindh should be viewed not just as the “American Taliban” but also as the “American al Qaeda," and warned about his release: “He hasn’t denounced radical Islam. And I think whether it’s the United States or the rest of the world — that should scare everybody.”

At least two U.S. senators have also spoken up about Lindh leaving prison, saying that he remains a danger to society and was among a number of terrorist prisoners who “continue to openly call for extremist violence.”

Earlier this year, Judge T. S. Ellis set Lindh’s release from Terre Haute, Ind., for Thursday. The special conditions of his supervised release include his internet communications being monitored, not being allowed to communicate with any extremists or to view extremist materials, and not being allowed to leave the U.S. without permission.