John Walker Lindh, the American captured in 2001 fighting for the Taliban, has been released early from federal prison, instantly drawing the fury of senior US government officials who say he may still pose a security risk.

Filmed by a CNN news crew in northern Afghanistan during his battlefield capture shortly after the 9/11 attacks, the footage of the prisoner who would become known as the “American Taliban” captivated the US.

Lindh, now 38, was released from federal prison in Terre Haute,Indiana, on probation on Thursday, after serving 17 years of a 20 year sentence for pleading guilty to fighting alongside the Taliban in Afghanistan.

His sentence reflected a plea deal that saw some of the more serious charges against him dropped amid allegations he was mistreated in custody. Lindh was present when a group of Taliban prisoners launched an attack outside the city of Mazar-e Sharif, leading to the death of CIA agent Johnny Spann.

A Bureau of Prisons spokesperson confirmed to the Guardian that Lindh had been released early due to “good conduct” but would not elaborate on the conditions of his parole.

Reports have indicated Lindh will be subject to a number of strict conditions during his three-year probation. He will be banned from possessing any “internet capable device” without permission and will be subject to constant monitoring. He will not be allowed to communicate in any language other than English and will be prevented from viewing any “material that reflects extremist or terroristic views”.

Lindh’s lawyer, Bill Cummings, declined a request for comment from the Guardian.

John Walker Lindh was 20 years old when he travelled to Afghanistan to join the Taliban. Born in California to an affluent San Francisco family, he converted to Islam as a teenager after watching the film Malcolm X. He journeyed to Yemen to learn Arabic and later admitted to meeting Osama bin Laden on at least one occasion.

Lindh’s early release drew the fury of senior government officials. The US secretary of state, Mike Pompeo, described it as “unexplainable and unconscionable”. A bipartisan group of US senators wrote to the federal bureau of prisons demanding to know how Lindh had qualified for early release.

“Our highest priority is keeping America safe, secure, and free,” the senators wrote. “To that end, we must consider the security and safety implications for our citizens and communities who will receive individuals like John Walker Lindh …”

Lindh claimed at trial he had not supported violence against America: “I never understood jihad to mean anti-Americanism or terrorism,” he told a Virginia court during his sentencing. But there are multiple indications he has not renounced extremism during his incarceration.

According to a 2016 intelligence report, leaked to Foreign Policy magazine in 2017, the prisoner “continued to advocate for global jihad and to write and translate violent extremist texts”. The report also claims Lindh told a TV news journalist in March 2016 “that he would continue to spread violent extremist Islam upon his release”.

Another account published on Thursday by the Atlantic, indicates Lindh has expressed support for Islamic State, and requested reading materials related to the terror group while incarcerated.

The 37-year-old’s case illustrates a wider issue in the US prison system with regards to inmates convicted of terror charges. There are more than 400 inmates currently held in the federal prison system with connections to international terrorism, with up to a quarter of those scheduled for release within the next five years.

In contrast to many European countries, the US has done little to invest in the rehabilitation of radicalized inmates.

Nonetheless, Lindh’s parents have continued to maintain their son is not a violent extremist.

Writing for the Observer in 2011, Lindh’s father, Frank Lindh, described his son as a “gentle young man”.

“John is blessed with a calm and curious nature. As a child, he was more sceptical than our other two children about such things as Santa Claus. When he was 12 years old, he saw the film Malcolm X, and was moved by its depiction of the pilgrims in Mecca. He began to explore Islam and, four years later, decided to convert,” Lindh wrote of his son.

“To me and to John’s mother, his conversion was a positive development and certainly not a source of worry.”

Frank Lindh did not respond to multiple requests for comment on Thursday.