Guantánamo, Cuba - A prisoner from the Guantánamo Bay detention center has been repatriated to Saudi Arabia, the Pentagon said on Wednesday, after being imprisoned for almost 21 years without being convicted of a crime.

Ghassan Al Sharbi was transferred to his home country after it was assessed that he no longer posed a threat to the national security of the US, the Department of Defense (DoD) said in a statement.

The 48-year-old Saudi national's repatriation was "subject to the implementation of a comprehensive set of security measures including monitoring, travel restrictions and continued information sharing," the DoD said.

According to Human Rights Watch, Al Sharbi was detained on March 28, 2002, in Faisalabad, Pakistan. Three months later, he was taken to Guantánamo via "extraordinary rendition."

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The Guantánamo Bay detention facility in Cuba now has 31 detainees left, according to the DoD, after it once held nearly 800 people.

The notorious camp was established by the George W. Bush administration after the September 11, 2001, terrorist attacks, to hold suspects without trial or even formal charges in some cases.

Human rights organizations have long called for the prison camp to be shut down. It has a reputation for reports of horrific torture, and inhumane treatment of detainees, and many other violations of international law.

Al Sharbi, an engineer, was suspected of having built bombs for the al-Qaeda terrorist organization. However, he was never convicted, according to the New York Times.

US authorities were interested in him because he had taken flight classes in Phoenix, Arizona, with two of the September 11 hijackers.