(NewsNation) — A Saudi prisoner at the Guantanamo Bay detention center who was suspected of trying to join the 9/11 hijackers has been sent back to his home country for treatment of mental illness, the Department of Defense said.

Mohammad Ahmad al-Qahtani was flown back to Saudi Arabia, to a treatment facility, from the U.S. base in Cuba after a review board including military and intelligence officials concluded he could be safely released after 20 years in custody.

The 46-year-old prisoner has suffered from mental illness, including schizophrenia, since childhood, according to medical examinations and records obtained by his lawyers. The U.S. dropped plans to try him after a Bush administration legal official concluded he had been tortured at Guantanamo.

With his release, there are now 38 prisoners left at the detention center. He is the second released under President Joe Biden, who has said he intends to close the facility.

National security lawyer Jason Wright said al-Qahtani’s release is in line with the Geneva Conventions.

“The U.S., like other countries in the world, is obligated under the Geneva Conventions from an international legal framework to follow the laws of war,” Wright said. “And here, there was a decision that continued detention was no longer necessary — consistent with the laws of war.”

However, Wright said it is unlikely al-Qahtani will ever truly be a free man.

“According to the order that I saw, he will be in a custodial psychiatric hospital. He will not be permitted to leave the country; he will be monitored,” Wright said.

In August 2001, al-Qahtani was turned away from the U.S. at the Orlando airport by immigration officers who were suspicious of his intentions. The lead Sept. 11 hijacker, Mohammed Atta, was going to pick him up to take part in the plot, according to previously released documents.

U.S. forces later captured him in Afghanistan and sent him to Guantanamo, where he was subjected to brutal interrogations that the Pentagon legal official in charge of war crimes commissions said amounted to torture.

That treatment included beatings, exposure to extreme temperatures and noise, sleep deprivation and extended solitary confinement. An FBI official in 2002 observed al-Qahtani speaking to non-existent people, hearing voices and crouching in a corner of his cell while covering himself with a sheet for hours at a time.

The 38 remaining prisoners at Guantanamo include 19 who have been approved for repatriation or resettlement by the review board. There are an additional seven who are eligible for review. Ten prisoners face trial by military commission, including five charged with involvement in the Sept. 11, 2001, attacks. Their death penalty case has been stalled for years in the pretrial phase.

The remaining two prisoners at the base have been convicted, one of whom, former Maryland resident Majid Khan, is nearing the completion of his sentence under a plea deal.