Notorious “American Taliban” John Walker Lindh, who was released from prison Thursday, has expressed support for ISIS, saying the terror group was “doing a spectacular job,” according to a newly released letter.

“The Islamic State is clearly very sincere and serious about fulfilling the long-neglected religious obligation to establish a caliphate through armed struggle, which is the only correct method,” Lindh wrote in a 2015 letter from prison to NBC’s KNBC in Los Angeles

The 38-year-old, who was captured fighting with the Afghan Taliban shortly after the 9/11 attacks, is expected to be sprung Thursday from a prison in Terre Haute, Indiana, after serving more than 17 years.

His plea deal called for a 20-year sentence, but he was released a few years early for good behavior amid concerns by US authorities that he remains a potentially violent extremist.

Lindh expressed remorse and denounced terrorism when he was sentenced in 2002, saying he did not support terrorism and that he had made a “mistake by joining the Taliban.”

But in his correspondence to the LA station, he expressed vastly different sentiments, saying he was proud “to take part in the Afghan jihad,” according to NBC News. In his letters, he signed his name as Yahya.

Among those who oppose his release is the family of CIA officer Johnny Spann, who was killed in Afghanistan during an uprising of Taliban prisoners. Spann had interrogated Lindh shortly before the attack.

Lindh expressed his sentiment about ISIS in response to a question from KNBC about whether the terror group represents Islam after the jihadists had beheaded Americans, including journalist James Foley in 2014.

In his fourth and final letter to the station, Lindh said he would no longer respond to a reporter’s question about ISIS.

Based on Lindh’s correspondence with journalists and other statements he made behind bars, the National Counter Terrorism Center said in a 2016 intelligence document that he “continued to advocate for global jihad and to write and translate violent extremist texts.”

A US official told NBC News that Lindh will live in Northern Virginia, where he will be subject to strict monitoring of his internet use, banned from international travel and undergo mental health counseling.

“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,” former US intelligence official Seamus Hughes told NBC News.

Although the terms of his supervised release last three years, officials told the network that the FBI is likely to keep a close eye on him.

Worries about Lindh’s radical views were the subject of a 2017 piece in Foreign Policy magazine, but they have not been widely publicized.

One expert said Lindh should clarify his views about ISIS.

“John Walker Lindh served his time. Given the support for ISIS expressed in this letter from four years ago, it would be important for Lindh to go on record declaring his intentions to live a peaceful and constructive life and to renounce terrorism and violence,” said Karen Greenberg, director of Fordham University’s Center on National Security, told NBC.

“Without that, allegations, confusion and anger will likely continue to surround him.”