American John Walker Lindh, who was captured fighting for the Taliban in Afghanistan in November 2001, is due to be released from prison on May 23.
Lindh, 38, known as the 'American Taliban,' is being freed from a high-security prison in Indiana three years early for good behavior in a 20-year sentence.
Lindh, born in California, converted to Islam at 16 and joined the Taliban in mid-2001.
After the United States invaded Afghanistan following the September 11, 2001, attacks, Lindh was one of hundreds of Taliban fighters captured on November 25, 2001.
On the eve of his release, U.S. television station KNBC said it had received letters from Lindh during his imprisonment in which he allegedly praised the Islamic State extremist group and called himself a political prisoner.
It was unclear why the letters, dating back to 2014-15, only came to light now.
Lindh, who gained Irish citizenship while in prison, cannot obtain a passport or travel abroad.
Lindh's release underscores the fact that, almost two decades later, the U.S. war against the Taliban continues.
Veteran U.S. diplomat Zalmay Khalilzad has been conducting several rounds of peace talks with the militants.
But the Taliban refuses to speak to the government in Kabul and has not renounced violence.
Khalilzad on May 22 briefed U.S. lawmakers in Washington on the progress of the talks.
The meeting was classified, but media reports suggested that the results of Khalilzad's efforts were met with skepticism on Capitol Hill.
Based on reporting by AP and AFP
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