The Trump administration has repatriated a longtime inmate at the Guantanamo Bay military prison to Saudi Arabia, where he will serve out the remainder of his 13-year sentence in connection with a 2002 attack on a French ship, the Pentagon said Wednesday.

The transfer of Ahmed al-Darbi to Saudi custody marks the first time the Trump administration has authorized the departure of an inmate from the facility at Guantanamo Bay, Cuba, which President Trump has promised to keep open and said could even house new detainees.

The move is unlikely to mark a shift in administration policy regarding prisoner transfers, which Trump has suggested threaten U.S. security.

Darbi pleaded guilty in 2014 to war crimes in relation to what military prosecutors characterized as plots to attack international ships in the Middle East. While he had been captured by the time the French oil tanker MV Limburg was attacked in October 2002, he was charged in connection with that assault. A Bulgarian crew member was killed in the attack.

The detainee is the brother-in-law of a Sept. 11, 2001, hijacker, Khalid Almihdhar, who helped commandeer the plane that crashed into the Pentagon.

Darbi has cooperated with U.S. authorities. According to his attorney, Ramzi Kassem, a professor of law at the City University of New York, Darbi had taken part in more than 600 interviews and given eight days of sworn testimony as a cooperating witness.

“Although his American captors tortured him cruelly and imprisoned him without charge for many years, my client provided unprecedented cooperation to the U.S. government and accepted responsibility for his own actions. The result for Mr. al-Darbi will be a total of 25 years in prison, while the men who actually carried out the Limburg attack received sentences of 10 years,” Kassem, who has served as Darbi’s lead defense counsel since 2008, said in a statement. “This is what passes for justice at Guantanamo.”

Darbi has been at Guantanamo since August 2002, according to military documents made public by WikiLeaks.

Kassem welcomed the transfer, which he said would reunite his client with his family. He said Darbi would serve out the remainder of his sentence — about nine years — in Saudi custody.

In a statement provided through Kassem, Darbi said: “To leave Guantanamo is to be born again. . . . I am deeply grateful to my country, Saudi Arabia, for welcoming back its son.”

He called on Saudi authorities to concern themselves with the plight of two fellow Saudis at Guantanamo, Mohammed al-Qahtani and Ghassan al-Sharbi.

The outlook is much more bleak for the 40 other inmates remaining at Guantanamo Bay.

While the Obama administration sought to reduce the prison population through overseas transfers — it resettled 196 prisoners — Trump has said “no further releases” should take place.

“These are extremely dangerous people and should not be allowed back onto the battlefield,” Trump tweeted shortly before taking office.

In January, Trump signed an executive order keeping the prison open and asking the Defense Department and other agencies to draft recommendations for how the United States should handle terrorism suspects captured overseas, including the question of whether and how they might be sent to Guantanamo.

Julie Tate contributed to this report.