The Pentagon on Wednesday announced the transfer of a prisoner at the Guantanamo Bay detention facility in Cuba to Saudi Arabia, where he will serve the remainder of his 13-year sentence.

In a statement, it said 43-year-old Ahmed Mohammed Ahmed Haza Al Darbi, who allegedly pleaded guilty to helping plan an attack in 2002 on a French oil tanker, has waived his right to appeal.

“In accordance with statutory requirements, the secretary of defense informed Congress of the United States’ intent to transfer this individual and of the secretary’s determination that this transfer met the statutory standard,” it said in the statement.

With Al Darbi’s transfer, 40 detainees remain at Guantanamo Bay.

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With the abrupt transfer announcement, Al Darbi has become the first detainee to leave the formidable wartime prison under President Donald Trump, who decided to keep the prison open and has vowed to fill it back up.

The statement also noted that the US coordinated with the Saudi government to ensure the transfer took place in accordance with established standards for security and “humane treatment”.

There are many reports that prisoners at the facility have faced unlawful torture, and some have not been able to obtain access to adequate medical treatment.

The Guantanamo Bay prison was opened shortly after the US invaded Afghanistan following the Sept. 11, 2001 terrorist attacks.

The first terrorism suspects were detained and taken to the prison on January 11, 2002 under the order of then-President George W. Bush.

Former US President Barack Obama promised to shut down the controversial prison after taking office in 2009 but was met with stiff opposition in Congress.