In a first since President Biden took office, the Biden administration has transferred a detainee at the U.S. military prison in Guantánamo Bay, Cuba, to Morocco, signaling a renewed effort to shrink the highly controversial prison's population — and possibly close it entirely.The transferred prisoner, 56-year-old Moroccan citizen Abdul Latif Nasser, had been cleared for release by a parole-like board in 2016, but was held at Gitmo for another five years. In total, he spent 19 years at Guantánamo without being charged, making him one of Gitmo's so-called forever prisoners subject to indefinite detention.Nasser's transfer was approved near the end of the Obama administration, which had pledged to shut down Guantánamo, but President Trump entered office before Nasser could be released. His arrival at the White House essentially brought the prisoner transfer process to a halt, with Trump instead vowing to "load [Guantánamo] up with some bad dudes."But the Biden administration has quietly begun to pick up where Obama left off by clearing additional prisoners for transfer and, now, sending one home."We're tremendously relieved," said one of Nasser's attorneys, Thomas A. Durkin of Chicago, who has represented him for more than a decade. "There were some pretty dark hours here and we didn't think that it was ever going to happen, which is a pathetic statement about our justice system, but we're very grateful that he's been released."Nasser's transfer to his home country leaves 39 men imprisoned at Guantánamo, down from nearly 800 since the prison opened in 2002. The majority of the remaining detainees — about three-quarters — are also "forever prisoners" being held without charge or trial. Ten of them have been cleared for release by Guantánamo's Periodic Review Board, as Nasser was, but the U.S. has yet to find countries willing to take them, subject to security assurances.The Obama administration official who oversaw prisoner transfers from Guantánamo, Lee Wolosky, told NPR that it is past time for that process to be revived."We're really in a different world than we were in 2001 when the 9/11 attacks happened and 2002 when Guantánamo was opened," Wolosky said. "The base in Afghanistan for al-Qaida no longer exists and the threat environment is just fundamentally changed, so it's really time to wrap up this chapter of our history and to move on."The Biden administration should "prosecute the individuals, finally, who we can prosecute...and we should release those that we're not going to charge," Wolosky added. "I would note that we still haven't been able to bring the 9/11 conspirators to trial, which is, frankly, a national embarrassment."Nasser's release comes amid additional signs that a change of policy approach is underway at Gitmo. Earlier this month, the chief prosecutor of the military court, Army Brig. Gen. Mark Martins, who also oversees the criminal case against the five men charged in the Sept. 11, 2001, terror attacks, made a surprise announcement that he will be retiring in September. And the chief defense counsel in the 9/11 case, Marine Brig. Gen. John Baker, will be retiring in November.Nasser, whose story was the subject of a Radiolab podcast series, landed in Morocco early Monday morning, was being held at a Casablanca police station, will rejoin his family in the next few days, and will work at his brother's swimming pool cleaning business, according to his lawyers.Durkin, one of Nasser's attorneys, said he has not yet spoken with Nasser since his release, but has talked with Nasser's brother by telephone."It was just one of those great moments in a lawyer's life, where the person on the other end is just ecstatic," Durkin said. "That said, it's a tragedy that he was there for 19 years without ever being charged. Not just a tragedy; it's a tremendous black mark for this country, as far as I'm concerned."In an interview with NPR, another attorney for Nasser, Columbia Law School professor Bernard Harcourt, called Nasser's release "a really important first step" in the Biden administration's stated goal of closing Guantánamo's prison.However, in a statement, Nasser's lawyers added: "It is hardly cause to celebrate the release of a man held for nineteen years without ever being charged with a crime, the last four of which were the collateral damage of the Trump Administration's and zealous Republican War on Terror hawks' raw politics. If this were a wrongful conviction case in Cook County, it would be worth $20 million. Nevertheless, we applaud the Biden Administration for causing no further harm."Some Republican senators continue to oppose the closure of Guantánamo's prison and the release of its prisoners, saying they remain a threat to the United States.Still, Wolosky, the former special envoy for Guantánamo under President Obama, said that "going forward, I would expect to see more deals done of this type," especially involving prisoners already cleared for release."Those are the easy ones for the Biden administration now to just pick up the old deal, dust it off...and get those deals finally completed after an unfortunate four-year delay during the Trump administration," Wolosky added.For Guantánamo prisoners not facing criminal charges but not yet cleared for release, State Department officials will likely begin negotiations with other countries willing to take them, subject to security measures.For prisoners facing charges, including the five 9/11 defendants, those cases could be settled — such as with guilty pleas in return for life in prison — as a less expensive, more efficient alternative to going to trial and facing the death penalty.In May 2018, one prisoner was transferred out of Guantánamo by the Trump administration, but that was part of a deal the prisoner made in which he agreed to plead guilty and testify against a fellow inmate in exchange for being sent to Saudi Arabia to finish his sentence there; it was not a part of a larger effort to reduce Guantánamo's prison population."So in certain respects," Wolosky said, "that one doesn't really count as a transfer that was made as a policy decision, as opposed to implementing a legal agreement." 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