As Steve Vladeck noted last week, lawyers for Guantánamo detainee Mohamedou Ould Slahi filed a motion in his habeas corpus case demanding that the Obama administration provide Slahi with a hearing before the interagency body known as the Periodic Review Board (PRB). (Disclosure: I am one of Slahi’s lawyers). Slahi’s motion highlights broader concerns about how the PRB is being implemented and the implications for Obama’s pledge to close Guantánamo as his presidency nears its end.

Obama established the PRB in March 2011 through Executive Order 13,567 to determine whether the continued detention of individual Guantánamo detainees is necessary to protect against “a significant threat to the security of the United States.”

So whereas habeas corpus proceedings test whether an individual is subject to law-of-war detention under the 2001 Authorization for Use of Military Force (AUMF) (e.g., because he was “part of” or “substantially supported” a covered enemy force at the time of capture), the PRB determines whether that individual should continue to be held even if he was initially eligible for AUMF detention.

Slahi’s case illustrates the relationship between the two proceedings. In 2009, DC district judge James Robertson granted Slahi habeas relief, concluding that his detention under the AUMF was unlawful because he was not “part of” al-Qaeda, as the government had alleged. In 2010, the DC Circuit vacated that ruling, concluding that the district court applied the incorrect legal standard for AUMF detention and remanding the case for additional fact-finding. The habeas challenge is still pending in the district court.

The PRB, on the other hand, is supposed to determine whether an individual, such as Slahi, should continue to be held, even if he was subject to AUMF detention at the time of capture. The PRB states that the prisoner must pose a significant threat to the United States for his detention to continue.

The Executive Order specifies that a detainee receive a PRB one year from the date the order was issued, at the latest. All PRBs, therefore, should have been conducted by March 2012.

But many detainees, including Slahi, have still not received a PRB. Thus far, the administration has held PRB hearings for only 14 detainees. Four more detainees have been notified of consideration for a hearing date (and one of those detainees has been given a hearing date). That leaves 49 additional detainees who are eligible for a PRB, either because they are slated for indefinite law-of-war detention or referred for trial. At the present rate, PRB hearings will not be completed for all eligible detainees until 2020. (These statistics are set forth in this helpful fact sheet published by Human Rights First).

The administration’s failure to provide detainees with a PRB highlights two problems with Obama’s Guantánamo. First, the delay conflicts with President Obama’s commitment to close Guantánamo. The PRB plays an important role in closing the prison because it is the administration’s designated mechanism for determining whether the US should continue to hold prisoners it has detained under the AUMF. If the Pentagon were faithfully implementing Obama’s priority of shutting down Guantánamo, presumably it would not be dragging its feet and would instead aggressively utilize the mechanism the President created to help move prisoners out of Guantánamo.

The PRB, moreover, is now even more critical in light of the DC Circuit’s decision last week in Bahlul v. United States, which held that military commissions cannot prosecute conspiracy under the Constitution because it is not a war crime under international law. This decision follows the DC Circuit’s earlier ruling that military commissions cannot prosecute material support for terrorism for the same reason. Together, these inchoate offenses, which do not require direct participation in a terrorist attack, provide the only potential charge for a number of the remaining Guantánamo detainees the administration said were eligible for prosecution (only 36 at the time of the Guantánamo Review Task Force’s 2010 report, and far fewer today).

If the Bahlul ruling stands, therefore, it will further limit the number of detainees who may be prosecuted and increase the number facing indefinite detention under the AUMF, all of whom must receive a PRB to determine whether they should continue to be held. And that determination — unlike a criminal trial — is predicated on whether the individual poses a threat going forward, not on what he did in the past.

Second, the failure to provide detainees with a PRB highlights the Obama administration’s tendency to cherry-pick concepts from international law, reaping the benefits (the power to detain until the end of hostilities) while dispensing with the limitations where convenient (the requirement of continued review to determine future dangerousness).

International humanitarian law imposes significant constraints on the continued detention of unprivileged belligerents, as the government alleges Guantánamo detainees to be, absent criminal charges. The Fourth Geneva Convention requires that unprivileged belligerents receive regular periodic review to determine whether their continued detention is imperative to security. This principle, as ICRC Senior Legal Adviser Jelena Pejic notes, should be even more stringently observed in noninternational armed conflicts since human rights law, which applies in this context, rejects the notion of indefinite detention. Continued review is also supported by the Constitution’s Due Process Clause, which, as the Supreme Court has indicated, requires a periodic individualized determination of future dangerousness for those held without charge for security purposes.

The PRB creates the appearance that Guantánamo detentions adhere to law of war principles by supplying a process for continued review. But the administration has essentially treated this process as discretionary by neglecting to provide the required review in a number of cases.

Creating a process is only the first step. That process must also be implemented faithfully, consistently, and in accordance with the legal principles that govern it. In this respect, the failure to provide Slahi and other detainees with a PRB reinforces one of the troubling features of Obama’s Guantánamo: the veneer of legality on paper, but arbitrariness on the ground.