WASHINGTON — The Pentagon on Monday announced that it had repatriated a prominent Guantánamo detainee who wrote a best-selling memoir recounting his abuse by American interrogators. The transfer reduced the remaining detainee population to 60.

The ex-detainee, Mohamedou Ould Slahi, 45, was transferred to his native Mauritania on Monday, officials said. A parole-like review panel of six agencies recommended his transfer in July, citing his “highly compliant behavior in detention” and “clear indications of a change in the detainee’s mind-set.”

“I feel grateful and indebted to the people who have stood by me,” Mr. Slahi said in a statement released by the American Civil Liberties Union, which helped to represent him. “I have come to learn that goodness is transnational, transcultural and transethnic. I’m thrilled to reunite with my family.”

Born in Mauritania, Mr. Slahi studied electrical engineering in Germany and then joined Al Qaeda in the early 1990s, when Osama bin Laden’s mujahedeen fighters were helping the anti-Communist resistance in Afghanistan, backed by the United States, after the Soviet invasion.