(CNN) 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, authorities said.

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, the Bureau of Prisons confirmed, 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

Because he was not native to Afghanistan and did not speak the local languages, Lindh told investigators that he joined the "Arab group," or al Qaeda, studying maps and explosives, fighting on a front line, and at one point, meeting with Osama bin Laden.

When US troops first encountered Lindh in November 2001, just weeks after the September 11 attacks, he was bedraggled and injured.

A CNN camera filmed as Lindh, a daze cast over his dirty face, told American forces how he had wound up at a detention camp in northern Afghanistan and survived a Taliban uprising there that killed hundreds of prisoners and a CIA officer, Johnny Michael Spann.

Lindh admitted to participating in the revolt near Mazar-e Sharif, Afghanistan, but prosecutors did not say that he had a role in Spann's death.

Initially charged with a raft of serious offenses, including conspiracy to kill US nationals, Lindh, in 2002, struck a deal reportedly offered by prosecutors in part to prevent details of the apparent mistreatment of Lindh at the hand of US forces by his defense. Lindh pleaded guilty to fighting alongside 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.

According to the NCTC report, as of May 2016, Lindh "continued to advocate for global jihad and to write and translate violent extremist texts." In March 2016, the report says, he "told a television news producer that he would continue to spread violent extremist Islam upon his release."

Lindh had made "pro ISIS statements to various reporters," the Bureau of Prisons report also stated.

In an email to his father included in the BOP report, Lindh said that he was "not interested in renouncing my beliefs or issuing condemnations."

The two assessments do not provide details for the statements, and the BOP and the NCTC declined to comment to CNN on the reports.

On Wednesday, a local NBC News station in Los Angeles released excerpts from correspondence a producer there had with Lindh from behind bars, where Lindh said in 2015 he thought ISIS was doing a "spectacular job."

"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 told the producer, according to the report.

Lindh denied a request by CNN to be interviewed in prison and his lawyers declined to comment on the counterterrorism assessments.

Prison term

In prison, Lindh was known to be deeply religious -- he recited the entire Quran from memory each week, and regularly gave a call to prayer for the other Muslims in his unit, according to a narrative written by an inmate who served with him.

Lindh went by the name Yayha, the inmate wrote in the anonymous essay, which was published by CAGE, a group started by someone released without charges after being detained in Guantanamo that advocates for those arrested or prosecuted in the war on terror. The human rights group Amnesty International cut ties with CAGE because of some of its statements and relationships with terror suspects.

"His whole life revolves around reading, writing, praying, and working out in his cell. His Muslim brothers know he is busy so they don't hesitate to cook for him in order make sure he eats well," the inmate wrote.

Lindh discussed his values in his own essay, published by CAGE in 2014 and titled "Memorising the Qur'an: A Practical Guide for Prisoners."

"Free time is a great gift from Allah and few people enjoy more of it than prisoners," Lindh wrote. "The best way we can express our gratitude to Allah for this gift is through the study, recitation, memorisation, contemplation, and implementation of His Noble Book."

On Monday, Johnny Spann, the father of the CIA officer killed in the Taliban uprising that Lindh participated in, petitioned the Virginia judge overseeing Lindh's case to investigate the extremist comments he allegedly made while in prison.

"You need to find out for sure, is this guy still the same al Qaeda member we put in jail? If he is still the al Qaeda member we put in jail then we need to throw the plea agreement away and do something else," Spann told CNN in an interview.

Spann has protested Lindh's early release to lawmakers, including Sen. Richard Shelby, an Alabama Republican, who said last month that he raised the issue with the White House.

In a tweet, Shelby wrote that President Donald Trump agreed that Lindh should serve his full sentence. Lindh's early release this week appears to be the result of time taken off of his sentence for good behavior.

The White House did not respond to a request for comment on this story, and legal experts question what power the President could have to prevent Lindh's release outside of a wider regulation change, which would likely invite a backlash.

Feds not prepared, experts say

After he leaves prison, Lindh's actions will be closely watched as part of a sweeping set of conditions imposed on his three years of supervised release by Judge T.S. Ellis , who oversees his case in Virginia.

Under the conditions, Lindh is not allowed to possess any "internet capable device" without permission from the probation office, and any approved device would be "monitored continuously."

Lindh is not allowed to have any online communications in any language other than English unless otherwise approved, cannot communicate with a known extremist, and cannot possess or view "material that reflects extremist or terroristic views."

The strict impositions are likely to be mirrored in dozens of cases like Lindh's. As of last week, there were 421 inmates with a history of, or nexus to, international terrorism, in the federal prison system, according to the Bureau of Prisons.

Some 60 to 100 of those prisoners are expected to be released in the next five years, said Mitch Silber, the former director of Intelligence Analysis at the NYPD, who now runs a nonprofit that works to rehabilitate radicalized individuals.

Counterterrorism experts have said that the federal government has not done enough to prepare for the coming trend of releases.

"Right now there is no program to rehabilitate and reintegrate them, give them the type of skills that will make it less likely they will recidivise to some type of criminality," Silber said.

Mary McCord, the former acting assistant attorney general for National Security at the Justice Department, said officials at the Justice Department had only begun to consider the impending releases during her tenure, which ended in 2017.

"At the time I left not nearly enough had been done and we were nervous about it," McCord said in an interview.

The Justice Department referred CNN to the BOP for comment, which said in a statement that it has policies to monitor communications of inmates with known or suspected ties to domestic and foreign terrorism and that they share information with law enforcement as appropriate.

The BOP also says it encourages all inmates, including those linked to terrorism, to participate in programs "that assist offenders in developing the skills necessary for a successful reintegration into society."

"Based on a series of focused interviews conducted by BOP staff, the BOP has found that many inmates have turned away from radicalized ideology in prison based on self-study, or due to participation in programming or sentence length," BOP said.

Silber, who founded his non-profit Parallel Networks with a former American al Qaeda sympathizer convicted of conspiring to solicit murder, said that prisons could adapt already existing re-entry programs for gang offenders to fit extremists.

He also noted that a dialogue about the origins of an extremist's turn to radicalization can be helpful in determining how to move away from an ideology.

In the absence of that, or in the case of a return to criminal behavior, federal investigators could open a new investigation into a released prisoner.

"Ultimately the hammer that the federal government has over (Lindh) is that if he does behave in a way that looks like he is turning toward terrorism, they can open up a new investigation on him and essentially we can begin the process again," Silber said.

Terms of release

In emails Lindh wrote from behind bars referenced in the BOP report, he expressed interest in moving post-release to Ireland, where he is a citizen, as well as Puerto Rico.

Lindh said that he had gained Irish citizenship in 2013 "as a result of having an Irish grandparent" and wanted to petition the country to take him in by explaining the "unique circumstances that make my survival in the US practically impossible."

"Essentially I am seeking asylum from one country where I am a citizen in another country where I am also a citizen," Lindh wrote in an email to CAGE, who he had asked to help him with his attempt to relocate to Ireland. A person familiar with Lindh's status confirmed he is an Irish citizen.

It appears likely that Lindh's plan to move to Ireland may be prevented at least for now by the terms of his supervised release, which say that he cannot hold a passport and can't leave the United States "without the express permission of this Court."

Prisoners leaving detention under terms of supervised release are required to send a release plan to the probation office of the district in the country where they want to go, who will investigate the case and decide whether or not to allow the residency.

CAGE's Outreach Director Moazzam Begg, who has spoken with Frank Lindh about his son's case, told CNN that it would make sense for Lindh to move back with his father in northern California.

"His father has always been a great supporter of him and has believed in his son's cause," Begg said.

Frank Lindh denied an interview request from CNN.