A longtime Guantanamo Bay prisoner who wrote a best-selling book about his experiences in the controversial military prison has been released to his home country of Mauritania, the Pentagon said Monday.

The release of Mohamedou Ould Slahi, believed to be the last inmate from Mauritania held at the facility in Cuba, brings the prison's remaining population down to 60.

His case became a cause celebre after the publication last year of "Guantanamo Diary," in which he outlines his treatment at the notorious US naval base in Cuba and says he was subjected to torture.

"The United States is grateful to the government of Mauritania for its humanitarian gesture and willingness to support ongoing US efforts to close the Guantanamo Bay detention facility," the US Defense Department said in a statement.

Slahi, 45, was detained in his home country of Mauritania following the September 11 attacks on the United States in 2001, on suspicion of involvement in an unsuccessful plot to bomb Los Angeles in 1999, and was taken to Guantanamo in August 2002 following interrogation in Jordan and Afghanistan.

In his book, Slahi described the toll of life inside the jail, saying: "I started to hallucinate and hear voices as clear as crystal. I heard my family in a casual familial conversation ... I heard Koran readings in a heavenly voice."

"I was on the edge of losing my mind," he added.

President Barack Obama wants to close the Guantanamo jail before he leaves office, but his efforts have faced stiff Republican opposition and time to shutter the prison is running out fast.

Still, the United States has in recent months accelerated the rate at which detainees who have been approved for transfer are released from the facility.

When Obama took office, there were 242 detainees at Guantanamo.