'American Taliban' John Walker Lindh released early for good behavior — despite reportedly pushing Islamic radicalism in prison

So-called "American Taliban" John Walker Lindh has been released from prison after 17 years of his 20-year sentence because of good behavior.

However, Lindh, who joined the Taliban in mid-2001, reportedly tried to spread Islamic radicalism while in prison. Also, multiple letters written by Lindh from prison expressed support for terror groups, including ISIS.

Who is John Walker Lindh?

Lindh, who is now 38 years old, traveled to Yemen in 1998 to study Arabic. He moved to Pakistan in 2000 and studied Islam. By the middle of 2001, he had joined the Taliban.

U.S. troops captured Lindh in Afghanistan after the Sept. 11 terror attacks.

What's happened now?

Lindh was released Thursday from a federal prison in Terre Haute, Indiana, CNN reported. He will remain on probation for three years, will have his internet usage monitored, and is banned from leaving the country. He will be living in northern Virginia.

Lindh said he regretted his actions when he was sentenced in 2002, but letters written by Lindh and sent by him to KNBC-TV paint a picture of a man who still supports terrorism. In one letter from 2015, Lindh said that ISIS was "doing a spectacular job" and that they were "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."

In another letter from 2014, Lindh said that he felt "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."

The Atlantic's Graeme Wood, who corresponded with Lindh while the latter was in prison, said that he believed Lindh had "converted from an al-Qaeda supporter" — but only to become a supporter of ISIS.

Also, Lindh has been accused of spreading radical Islamist propaganda to fellow prisoners, using rhetoric from formerly high-ranking Muslim Brotherhood member Sayyid Qutb.

The father of the first US victim of the Taliban in Afghanistan weighed in

Mike Spann was a CIA operative killed by Taliban prisoners who were trying to escape from U.S. custody. Although Lindh was reportedly not involved in Spann's death, Spann had interviewed him shortly before the incident took place.

Spann's father, Johnny, told ABC News that he felt like he had "to apologize" to his son for Lindh's release, "because I feel like we failed him." Spann called on President Donald Trump to stop Lindh from being released, begging, "Mister president, please do your job."