The last two prisoners to leave the U.S. detention center at Guantánamo Bay were dead. On February 1, Awal Gul, a 48-year-old Afghan, collapsed in the shower and died of an apparent heart attack after working out on an exercise machine. Then, at dawn one morning in May, Haji Nassim, a 37-year-old man also from Afghanistan, was found hanging from bed linen in a prison camp recreation yard.

In both cases, the Pentagon conducted swift autopsies and the U.S. military sent the bodies back to Afghanistan for traditional Muslim burials. These voyages were something the Pentagon had not planned for either man: Each was an “indefinite detainee,” categorized by the Obama administration’s 2009 Guantánamo Review Task Force as someone against whom the United States had no evidence to convict of a war crime but had concluded was too dangerous to let go. Today, this category of detainees makes up 46 of the last 171 captives held at Guantánamo. The only guaranteed route out of Guantánamo these days for a detainee, it seems, is in a body bag.

The responsibility lies not so much with the White House but with Congress, which has thwarted President Barack Obama’s plans to close the detention center, which the Bush administration opened on Jan. 11, 2002, with 20 captives.

Congress has used its spending oversight authority both to forbid the White House from financing trials of Guantánamo captives on U.S. soil and to block the acquisition of a state prison in Illinois to hold captives currently held in Cuba who would not be put on trial — a sort of Guantánamo North.

The latest defense bill adopted by Congress moved to mandate military detention for most future al Qaida cases. The White House withdrew a veto threat on the eve of passage, and then Obama signed it into law with a “signing statement” that suggested he could lawfully ignore it.

On paper, at least, the Obama administration would be set to release almost half the current captives at Guantánamo. The 2009 Task Force Review concluded that about 80 of the 171 detainees now held at Guantánamo could be let go if their home country was stable enough to help resettle them or if a foreign country could safely give them a new start.

But Congress has made it nearly impossible to transfer captives anywhere. Legislation passed since Obama took office has created a series of roadblocks that mean that only a federal court order or a national security waiver issued by Secretary of Defense Leon Panetta could trump Congress and permit the release of a detainee to another country.

Neither is likely: U.S. District Court judges are not ruling in favor of captives in the dozens of unlawful detention suits winding their way from Cuba to the federal court in Washington. And on the occasions when those judges have ruled for detainees, the U.S. Court of Appeals has consistently overruled them in an ever-widening definition of who can be held as an affiliate of al Qaida or the Taliban.

Meanwhile, Defense Department General Counsel Jeh Johnson, the Pentagon’s top lawyer, believes that Congress crafted the transfer waivers a year ago in such a way that Panetta (and Robert Gates before him) would be ill-advised to sign them. (In essence, the Secretary of Defense is supposed to guarantee that the detainee would never in the future engage in violence against any American citizen or U.S. interest.)