Dawn arrives at the now closed Camp X-Ray, top, which was used as the first detention facility for al-Qaida and Taliban militants at the Guantanamo Bay Naval Base, Cuba.
Why was the prison at Guantanamo created?
It was set up to hold suspects in the "war on terror" declared by then-U.S. president George W. Bush in the wake of the Sept. 11, 2001, attacks on New York and Washington, as well as to house U.S. military commissions that would prosecute these suspects.
The international organization Human Rights Watch notes that such proceedings "lack the due process protections of U.S. federal courts." As such, they permit prisoners to be held indefinitely and allow testimony derived through torture.
Why does Obama want it closed?
When he was a presidential candidate in 2008, Barack Obama said that he considered the existence of the Guantanamo facility to be a legal and moral abomination; that the military commissions' apparatus harmed the reputation of the United States as a state governed by the rule of law.
More recently, he has emphasized the radicalizing effect the existence of Gitmo has had on susceptible young men and the high cost on maintaining the operation – more than $5-billion (U.S.) and counting.
What stopped him?
Two things.
First, the President himself blinked. In May, 2009, four months after he signed the executive order to close the prison at the Guantanamo Bay Naval Base within one year, Mr. Obama backtracked. He announced his administration would continue to use the military commissions to prosecute suspects, but with improved rules of procedure. He added, to the delight of civil-liberty advocates, that there also would be the option of trying some of the accused in U.S. federal courts wherever feasible.
The prospect of such trials was largely dashed in 2010, however, when the administration's decision to prosecute five 9/11 suspects in New York was met by a backlash from the public and political establishment and the White House climbed down.
That's when the Democratic Congress stepped in and closed the U.S. court option by passing a defence bill that prevented the military from spending any of its funding on transferring Guantanamo inmates to the United States, even for the purpose of prosecution.
An increasingly Republican Congress is unlikely to revoke such measures, even for the last of the inmates.
"As far as I'm concerned, every last one of them can rot in hell," Arkansas Senator Tom Cotton said recently, "but as long as they don't do that, they can rot in Guantanamo Bay."
Republican presidential candidate Donald Trump has said that, if elected, he will fill Guantanamo with "bad dudes" and "bring back a hell of a lot worse than waterboarding."
Who has been held there?
At its peak, in June, 2003, there were 684 detainees in Guantanamo. A total of 779 were held there at some point between 2002 and today. Most were from Afghanistan, Saudi Arabia and Yemen, although suspects hailed from as many as 50 different countries.
They included suspects in the attacks on U.S. embassies in East Africa in 1998; the attacks on ships in the area of the Arabian Peninsula – including the 2000 attack on the USS Cole in Yemen that killed 17 U.S. sailors; the Sept. 11, 2001, attacks on New York and Washington; as well as many enemy combatants captured in Afghanistan and Iraq.
Who's left?
With the most recent transfer of detainees to the United Arab Emirates, just 61 inmates remain, 20 of whom have also been cleared for transfer out of Guantanamo.
Of the 41 not cleared, seven still are being prosecuted in trials that have lasted years and show no end in sight.
They include:
Abd al-Rahim al-Nashiri, who is accused of plotting the attack on the USS Cole;
Khalid Sheikh Mohammed, the accused architect of the Sept. 11, 2001, attacks;
Ramzi bin al-Shibh, who is alleged to be another senior al-Qaeda figure linked to the Sept. 11 attacks but failed to get a U.S. visa;
Mustafa Ahmed al-Hawsawi, the suspected financial chief used by Mr. Mohammed to arrange the funding for the Sept. 11 attacks;
Walid bin Attash, who allegedly helped in the preparation of the 1998 East Africa embassy bombings and acted as a bodyguard to al-Qaeda leader Osama bin Laden.
The lone Canadians at Gitmo
Abdurahman Khadr and his younger brother, Omar, were among Guantanamo's inmates.
Omar Khadr, who was captured in 2002 and detained for 10 years, is the more famous of the two. At the age of 15, he was involved in a firefight with U.S. soldiers at a village in Afghanistan and seriously wounded. He was accused of and pleaded guilty in 2010 to throwing a grenade that killed a U.S. serviceman. Sentenced to eight years in prison, he was repatriated to Canada in 2012 to serve the remainder of his sentence and was released on bail in May, 2015, pending an appeal of his U.S. conviction.
Abdurahman Khadr was also sent to Guantanamo in 2002 (at the age of 20) but appears to have been there as a mole for the U.S. Central Intelligence Agency. He was released in 2003 and has lived in Toronto for most of the past several years.