John Walker Lindh, the Californian who took up arms for the Taliban and was captured by US forces in Afghanistan in 2001, has been released from prison after more than 17 years under tight restrictions that reflected US government fears he still harbours radical views.
President Donald Trump reacted by saying, "I don't like it at all."
"Here's a man who has not given up his proclamation of terror," he said.
Lindh, 38, left a federal penitentiary in Indiana after getting time off for good behaviour from the 20-year sentence he received.
It was not immediately clear where the man known as the "American Taliban" will live or what he will do. He turned down an interview request last week, and his attorney declined to comment Thursday.
In an interview, Secretary of State Mike Pompeo decried Lindh's early release as "unexplainable and unconscionable" and called for a review.
The president said he asked lawyers whether there was anything that could be done to block Lindh from getting out but was told no.
Under restrictions imposed by a federal judge in Virginia, Lindh's internet devices must have monitoring software, his online communications must be conducted in English, he must undergo mental health counseling, he is forbidden to possess or view extremist material and he cannot hold a passport.
FBI counterterrorism officials work with federal prison authorities to determine what risk a soon-to-be-released inmate might pose.
Probation officers never explained why they sought the restrictions against Lindh. But in 2017, Foreign Policy magazine cited a National Counterterrorism Centre report that said Lindh "continued to advocate for global jihad and to write and translate violent extremist texts."
Lindh converted to Islam as a teenager after seeing the movie "Malcolm X" and eventually made his way to Pakistan and Afghanistan and joined the Taliban.
He met Osama bin Laden and was with the Taliban on September 11, 2001, when al-Qaida terrorists attacked the World Trade Centre and Pentagon.
Lindh was captured on the battlefield after the US invasion of Afghanistan following 9/11 and was initially charged with conspiring to kill Mike Spann, a CIA operative who died during an uprising of Taliban prisoners.
Lindh denied any role in Spann's death. But he admitted carrying an assault rifle and two grenades during his time with the Taliban.
The bureau defended itself Thursday in a statement that said Lindh's release followed federal laws and guidelines.
It said it works closely with outside agencies "to reduce the risk terrorist offenders pose inside and outside of prisons," and added no radicalised inmate has returned to federal prison on terrorism-related charges.
Australian Associated Press