Abdul Rabbani, who was born in 1967, is believed to have been one of the oldest inmates at the facility on a US base in Cuba.

US officials accused him of working for avowed 9/11 mastermind Khalid Sheikh Mohammed (KSM) and operating an Al-Qaeda safe house in Karachi, but his detainee assessment indicates he was not believed to have had "specific insight into Al-Qaeda operational plans."

Mohammed Rabbani, born in 1969, was accused of recruiting his older brother into extremist circles. He is believed to have organized travel and funds for KSM and Abd al-Rahim al-Nashiri, the mastermind of the October 2000 suicide bombing of the USS Cole missile destroyer, which left 17 US sailors dead.

The pair were arrested in Karachi in September 2002 by Pakistani authorities, according to a Senate intelligence committee report that also names Mohammed Rabbani as one of 17 detainees subjected to torture at overseas CIA secret prisons, known as black sites.

Both men arrived at Guantanamo Bay in 2004 and were approved for release in 2021, the Defense Department said in a statement.

Their release brings to 32 the number of detainees left at Guantanamo Bay. Of those, 18 are eligible for transfer, three are eligible for review, nine are on trial in US military commissions and two have been convicted.