Six hundred and twenty-one detainees have been transferred from Guantanamo Bay since 2002. NPR and The New York Times have identified at least a dozen of the 621 whom have resumed terrorist activities. Of that dozen, two became leaders of al Qaeda in the Arabian Peninsula. The news outlets acquired a large cache of secret documents pertaining to Guantanamo Bay from the Wikileaks windfall. They report that detainees have been transferred to 52 countries. Some transferees are later released by the receiving nation. Afghanistan has received the most 199 transfers, the most of any nation.
The first transfer was to the United States and occurred in 2002. The transferee, Yaser Esam Hamdi, was the subject of the 2004 Supreme Court case Hamdi v. Rumsfeld. Hamdi is a US citizen who was captured in Afghanistan in 2001 and treated as an enemy combatant. After US authorities found out Hamdi was an American citizen, they transferred him to a naval brig in Norfolk, Virginia. Hamdi's father challenged the detainment of his son without due process. Eight justices held that a US citizen cannot be held indefinitely without due process. Justice O'Connor's plurality opinion found that the government must charge detainees and give them a hearing. The plurality did not extend the requirements any further in regards to ongoing military conflicts. Hamdi was deported to Saudi Arabia following the Court's ruling.
The Combatant Status Review Tribunal [PDF] was a result of the Court's plurality opinion in Hamdi. In 2008 the Court ruled in Boumediene v. Bush that even though Guantanamo Bay is sovereign territory of Cuba, the fact that the US maintains jurisdiction over the territory entitles all detainees to habeas corpus, regardless of their citizenship. The release of Guantanamo Bay detainees remains highly controversial, as exemplified by the May 2014 exchange of several Guantanamo Bay detainees for US prisoner of war Bowe Bergdahl.