WASHINGTON—The Biden administration on Monday transferred a Guantánamo Bay detainee to his home country for the first time, a policy shift from the Trump presidency that repatriated a Moroccan man years after he was recommended for discharge. The prisoner, Abdullatif Nasser, who’s in his mid-50s, was cleared for repatriation by a review board in July 2016 but remained at Guantánamo under President Donald Trump. In announcing his transfer Monday, the Pentagon cited the board’s determination that Nasser’s detention was no longer necessary to protect U.S. national security. Nasser, also known as Abdul Latif Nasser, arrived Monday in Morocco, where police took him into custody and said they would investigate him on suspicion of committing terrorist acts—even though he was never charged while in Guantánamo. The State Department said in a statement the Biden administration would continue “a deliberate and thorough process” to reducing the detainee population at Guantánamo “while also safeguarding the …