The failure to close Guantánamo, where 80 detainees remain, is a shameful stain on Congress, which has hindered efforts to release prisoners and barred the Pentagon from moving those remaining to prisons in the United States. The prison has undermined America’s standing as a champion of human rights and set a deplorable example for other governments inclined to violate international human rights law. Its familiar orange jumpsuits have been made part of the terrorists’ propaganda, most recently by Islamic State fighters in photos and videos that show the execution of hostages.

There is a modest step still available to Mr. Obama to demonstrate to the world that he is willing to acknowledge what has taken place at Guantánamo. The United Nations special rapporteur who examines issues of torture has sought access to the detainees for years, seeking to document their treatment while in custody. The government has refused repeated requests since 2004, with no good reason.

“I want to believe that the use of torture by the United States is a dark chapter that has ended,” Juan Méndez, the special rapporteur, said in an interview. “But I can’t be certain of that until we see a change in policy and verify that the United States is meeting all its international obligations.”

The defense team of Ammar al-Baluchi, one of the detainees at Guantánamo who is being tried in connection with the 9/11 attacks, filed a motion in May asking the military commission to allow him to meet with Mr. Méndez. Thomas Pickering, a veteran diplomat who has served as ambassador to Russia, India and the United Nations in Republican and Democratic administrations, has filed a memorandum supporting this request. Mr. Pickering wrote that recent reports of “heavy-handed and even brutal force-feedings, indifferent medical care, unacceptably cold stainless steel cells, indefinite solitary confinement” at Guantánamo may constitute violations of the Geneva Conventions and the Convention Against Torture. The United States is a signatory of both.