WASHINGTON : A Mauritanian prisoner who wrote a best-selling memoir about his long ordeal at the US military prison at Guantanamo Bay has been returned to his home country, the U.S. Defense Department said on Monday.

Mohamedou Ould Slahi, author of “Guantanamo Diary,” was released after a review board determined that his continued detention was not necessary to protect against a threat to US security, the department said in a statement.

His departure reduced to 60 the number of prisoners held at the facility set up to hold terrorism suspects after the Sept. 11, 2001, attacks. Nineteen of them have been cleared for release.

Slahi arrived in Guantanamo in August 2002. He has been held without charge or trial, appearing before a multi-agency government panel that reviews detainees’ cases on June 2.

His hearing was part of President Barack Obama’s stepped-up effort to pare back the Guantanamo detainee population before he leaves office in January.

Obama promised to close the controversial detention center in Cuba when he became president in January 2009. That has proven difficult in the face of stiff opposition in the USCongress, mostly from Republicans.

Republican White House hopeful Donald Trump has called for expansion of the prison.

Slahi became one of Guantanamo’s most prominent inmates with the 2015 publication of a prison memoir in which he described years of detention and interrogation, including being subjected to harsh techniques widely considered torture.

Slahi was originally suspected of being a senior recruiter for al Qaeda. But his lawyers contended that his links to militants were limited to the early 1990s when he fought in Afghanistan with mujahideen anti-communist insurgents.

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