The United States announced Thursday it has repatriated a Moroccan prisoner who spent 13 years at Guantanamo Bay, as part of a long-running effort to close the military prison in Cuba.

The Pentagon said Younis Abdurrahman Chekkouri was sent home to Morocco following a "comprehensive" security review that saw six US departments and agencies unanimously approve him for transfer.

He was repatriated on Wednesday, said a Pentagon official, who would not disclose whether Chekkouri would be imprisoned, placed under house arrest or released.

"The United States coordinated with the Government of Morocco to ensure this transfer took place consistent with appropriate security and humane treatment measures," said Commander Gary Ross, a Pentagon spokesman.

The 47-year-old was captured by Pakistani forces in late December 2001 with a group of several dozen Arab fighters who had fled the Tora Bora cave complex in eastern Afghanistan, a stronghold of the Taliban and its Al-Qaeda allies.

Transferred to Guantanamo in May 2002, he was never charged.

The United States considered him a "close associate" of late Al-Qaeda leader Osama bin Laden and a founder of the Moroccan Islamic Combatant Group (GICM), according to a once secret US military file released by anti-secrecy group WikiLeaks.

The file said Chekkouri had recruited GICM fighters for Al-Qaeda in Afghanistan and that he had participated in combat against the US-led international coalition in Afghanistan in 2001.

GICM is considered responsible for deadly bombings in Casablanca in 2003 and Madrid in 2004.

Following Chekkouri's release, 115 detainees still remain in the prison opened to hold terror suspects following the September 11, 2001 attacks.

That's less than half the number -- 242 -- who were detained there when President Barack Obama took office in January 2009.

Obama has vowed to close the controversial facility but has struggled to do so in the face of domestic opposition at home in Congress and abroad from allies reluctant to take in one-time terror suspects.

He now has just a little more than a year to make good on his promise.

The Pentagon has said it is studying the cost of moving detainees to US military prisons at Fort Leavenworth in Kansas and Joint Base Charleston in South Carolina.