Mohamedou Slahi was sent home Monday to his native Mauritania, according to the American Civil Liberties Union, which led the campaign for his release.
A joint review board of US security and intelligence officials cleared Slahi for release in July after determining his detention is not necessary "to protect against a continuing significant threat to the United States," the Department of Defense's Periodic Review Secretariat
"I feel grateful and indebted to the people who have stood by me," Slahi said in a statement
through the ACLU. "I have come to learn that goodness is transnational, transcultural, and trans-ethnic. I'm thrilled to reunite with my family."
His case drew sympathy in the United States and abroad, thanks in large part to his critically acclaimed memoir, "Guantanamo Diary
." Based on more than 400 handwritten pages he shared with his lawyers, the first-person account recounted harrowing tales of beatings, sensory deprivation and starvation.
Slahi was detained by Mauritanian authorities in 2001 and transferred to Guantanamo Bay from Bagram Air Force Base in Afghanistan in 2002 under suspicion of being a member of al Qaeda.
He was accused of recruiting three September 11 hijackers and involvement in other terror plots in Canada and the United States. Slahi admitted to traveling to Afghanistan to fight in the early 1990s, when the United States was supporting the Mujahedeen in their fight against the Soviet Union. He pledged allegiance to al Qaeda in 1991 but said he broke ties with the group soon after.
He was never charged and his lawyers said there was very little evidence against him.
In "Guantanamo Diary," he described long days in isolation, chained to the floor in agonizing positions, often deprived of food and sleep in extreme temperatures. The book spent several weeks on The New York Times' best-seller list and led to several petitions calling for his release that earned more than 100,000 signatures, according to the ACLU.
After the book was published in 2015, the Pentagon pointed to reports by the Armed Services Committee and the Department of Justice that found that then-Secretary of Defense Donald Rumsfeld had signed off on such treatment as special interrogation techniques.
"We are thrilled that our client's nightmare is finally ending," said Nancy Hollander, one of Slahi's attorneys. "After all these years, he wants nothing more than to be with his family and rebuild his life. We're so grateful to everyone who helped make this day a reality."
With Slahi's release, 60 prisoners remain in Guantanamo, the Department of Defense said. Nineteen of them have been cleared for release, according to the ACLU.