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.

Key points: Donald Trump said he asked lawyers whether anything could be done to block Mr Lindh from getting out

Donald Trump said he asked lawyers whether anything could be done to block Mr Lindh from getting out Secretary of State Mike Pompeo said his early release was "unexplainable and unconscionable"

Secretary of State Mike Pompeo said his early release was "unexplainable and unconscionable" Mr Lindh met Osama bin Laden and was with the Taliban on September 11, 2001, when Al Qaeda terrorists attacked the World Trade Centre and the Pentagon

His release is under tight restrictions that reflect US Government fears he still harbors 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," Mr Trump said.

Mr Lindh, 38, left a federal penitentiary in Terre Haute, Indiana, after getting time off for good behaviour from the 20-year sentence he received when he pleaded guilty to providing support to the Taliban.

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.

John Walker Lindh converted to Islam as a teenager and eventually joined the Taliban. ( AP: J Scott Applewhite )

In a Fox News interview, Secretary of State Mike Pompeo decried Mr Lindh's early release as "unexplainable and unconscionable" and called for a review of prison system policies.

The President said he asked lawyers whether there was anything that could be done to block Mr Lindh from getting out but was told no. Mr Trump said the US would closely monitor him.

A federal judge in Alexandria, Virginia, imposed the following restrictions on Mr Lindh:

his internet devices must have monitoring software;

his internet devices must have monitoring software; his online communications must be conducted in English;

his online communications must be conducted in English; he must undergo mental health counselling;

he must undergo mental health counselling; he is forbidden to possess or view extremist material; and

he is forbidden to possess or view extremist material; and he cannot hold a passport or leave the US.

Lindh continued extremist views, according to media reports

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 Mr Lindh. But in 2017, Foreign Policy magazine cited a National Counterterrorism Centre report that said Mr Lindh "continued to advocate for global jihad and to write and translate violent extremist texts".

NBC reported on Wednesday that Mr Lindh, in a letter to a producer from Los Angeles-based affiliate KNBC, wrote in 2015 that the Islamic State group was "doing a spectacular job".

Mr 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 Qaeda terrorists attacked the World Trade Centre and the Pentagon.

Mr Lindh was captured on the battlefield after the US invasion of Afghanistan following the September 11 attacks and was initially charged with conspiring to kill Mike Spann, a CIA operative who died during an uprising of Taliban prisoners shortly after he interrogated Mr Lindh.

Mr Lindh denied any role in Mr Spann's death. But he admitted carrying an assault rifle and two grenades.

Mr Spann's daughter Alison Spann, now a journalist in Mississippi, posted a letter on Twitter that she said she had sent to Mr Trump.

In it, she called Mr Lindh's early release "a slap in the face" to everyone killed on September 11 and in the war on terror since then, along with "the millions of Muslims worldwide who don't support radical extremists".

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Republican Alabama senator Richard Shelby and Democratic New Hampshire senator Maggie Hassan also expressed concern about Mr Lindh's release in a letter last week to the Federal Bureau of Prisons.

"We must consider the security and safety implications for our citizens and communities who will receive individuals like John Walker Lindh who continue to openly call for extremist violence," they wrote.

The bureau defended itself on Thursday in a statement that said Mr Lindh's release followed federal laws and guidelines.

It said it worked closely with outside agencies "to reduce the risk terrorist offenders pose inside and outside of prisons", and added that no radicalised inmate had returned to federal prison on terrorism-related charges.

CAGE advocate says criticism of early release is misguided

Moazzam Begg, a former detainee at Guantanamo who now serves as director of outreach for London-based CAGE (which supports the rights of those accused of terror-related crimes), said the criticism over Mr Lindh's early release was misguided.

Mr Begg said if anything, Mr Lindh was imprisoned too long.

He noted that many of the other Taliban fighters who were sent to Guantanamo as enemy combatants were released much earlier.

As for Mr Lindh's letter in support of the Islamic State group, Mr Begg noted that it was written four years ago and that Mr Lindh might not have had full knowledge of the group's atrocities from behind bars.

"Nobody really knows what his views are right now in 2019," he said.

In a statement, Mr Begg said: "It is now time for him to be allowed to restart his life in peace and freedom."

Mr Lindh was with the Taliban on September 11, 2001, when Al Qaeda terrorists attacked the World Trade Centre and the Pentagon. ( AP: Alexandria Sheriff's Office )

AP