By Idrees Ali

WASHINGTON (Reuters) -President Joe Biden’s administration said on Monday that it had transferred its first detainee from the U.S. military prison at Guantanamo Bay, a Moroccan man imprisoned since 2002, lowering the population at the facility to 39.

Abdul Latif Nasir, 56, was repatriated to Morocco.

Set up to house foreign suspects following the Sept. 11, 2001, attacks on the United States, the prison came to symbolize the excesses of the U.S. “war on terror” because of harsh interrogation methods critics said amounted to torture.

While former President Donald Trump kept the prison open during his four years in the White House, Biden has vowed to close it, a promise White House press secretary Jen Psaki reiterated on Monday.

Nasir had been cleared for release in 2016 during the Obama administration before Trump took office. Most of the prisoners left at Guantanamo Bay have been held for nearly two decades without being charged or tried.

“The (Biden) administration is dedicated to following a deliberate and thorough process focused on responsibly reducing the detainee population of the Guantanamo facility while also safeguarding the security of the United States and its allies,” State Department spokesman Ned Price said in a statement.

Morocco’s general prosecutor said in a statement that Nasir would be investigated for suspected involvement in terrorist acts, and a police source said he had been taken into custody in Casablanca.

More than a dozen Moroccans have been held at Guantanamo Bay and those repatriated have faced investigation and trial. One, Ibrahim Benchekroun, was jailed for six years after being repatriated in 2005 and died in 2014 in Syria where he had traveled to join a militant group.

A senior U.S. administration official, speaking on condition of anonymity, said that of the remaining detainees at the prison, 10 are already eligible for transfer.

Advocacy groups welcomed the move but said more needed to be done.

“The Biden administration urgently needs to negotiate and implement similar decisions for other cleared prisoners,” said Hina Shamsi, director of the American Civil Liberties Union’s national security project.

“Bringing an end to two decades of unjust and abusive military detention of Muslim men at Guantanamo is a human rights obligation and a national security necessity,” Shamsi said.

Opened under Republican President George W. Bush, the prison’s population peaked at about 800 inmates before it started to shrink. President Barack Obama, a Democrat like Biden, whittled down the number, but his effort to close the prison was stymied largely by Republican opposition in Congress.

The federal government is barred by law from transferring any inmates to prisons on the U.S. mainland. Even with Democrats controlling Congress now, Biden has majorities so slim that he would struggle to secure legislative changes because some Democrats might also oppose them.

U.S. Secretary of State Antony Blinken said last month that the administration was actively looking into recreating the position of a State Department envoy for the closure of the prison at the Guantanamo Bay naval base.

(Reporting by Idrees Ali, additional reporting by Ahmed Eljechtimi in Rabat and Daphne Psaledakis in Washington; Editing by Tomasz Janowski and Howard Goller)