The brother of a prominent Guantánamo Bay prisoner was denied entry to the United States this weekend as he attempted a trip to advocate for his brother’s release.

Mohamedou Ould Slahi is one of the most famous of the 80 men left at Guantánamo. Last year, Guantánamo Diary, his brutal memoir of imprisonment and torture by the United States and its counterterrorism allies, became a bestseller. Held in Guantánamo for nearly 14 years without being charged with a crime, Slahi is scheduled to go before the prison’s Periodic Review Board on June 2. The interagency panel will review his case and could possibly recommend his release.

Mohamedou’s younger brother, Yahdih Ould Slahi, lives in Düsseldorf, Germany, and has been trying to secure his brother’s freedom for years. He was planning to come to the United States to meet with journalists and attend a series of public events ahead of the review board hearing.

Yet when Yahdih, a German citizen, arrived at John F. Kennedy Airport in New York on Saturday, May 21, he was immediately taken into custody by Customs and Border Patrol. He was held overnight, questioned for hours, and then sent back to Germany on Sunday evening.

“He was asked questions about his family, his brother, and what he knew about why his brother was in Guantánamo,” said Hina Shamsi, director of the National Security Project at the American Civil Liberties Union. “It was a harrowing, stressful, and exhausting experience.”

(Disclosure: I am moderating a panel discussion with Yahdih in Washington, D.C., this week, at the invitation of the ACLU.)

Asked by The Intercept about the decision to refuse Yahdih entry, the CBP sent a boilerplate statement about its broad authority to control admission to the United States.

Now 45, Mohamedou Slahi was picked up in his native Mauritania in 2002 and held in Jordan and Afghanistan before being brought to Guantánamo later that year. In his memoir, Slahi describes being held in isolation, beaten, exposed to freezing temperatures, and subjected to sleep deprivation, sexual humiliation, and threats against his family. Much of his account has been substantiated by government investigations into detainee abuse.

Slahi has admitted that he went to fight in Afghanistan with al Qaeda against the communist government in 1990, but he maintains that he has had no connection to the group since 1992. One military official refused to participate in a prosecution of Slahi after learning about his torture, and a former prosecutor has said he could not find anything with which to charge him. A federal judge ordered his release in 2010, but the decision was vacated on appeal and the case has stalled.

“Yahdih wanted this chance to speak in person to Americans, to thank his brother’s American supporters, and to talk about the impact of his brother’s detention on his family and why they want him released,” said Shamsi, of the ACLU. “Yahdih’s perspective is one that Americans rarely get to hear.”

The Periodic Review Board that will evaluate Slahi’s case in June is a critical part of the Obama administration’s attempt to move as many people as possible out of Guantánamo before the end of this year, even as it looks increasingly unlikely that Obama will fulfill his longtime pledge to close the facility entirely.

Congress has barred detainees from being brought to the United States, even for continued detention, and restricted the countries to which they can be sent. The Guardian reported Monday that the administration had secured deals with about six countries to accept about two dozen men by the end of July.

If those transfers occur, it would leave only a handful of the men who have so far been cleared for release. Seven prisoners currently face charges in front of the military commissions, 21 have been referred for prosecution, and another 21, including Slahi, fall into the category of so-called forever prisoners: those the administration has determined it can’t charge with a crime, but who it still believes could pose a threat to the United States if released. Both the men referred for charges and the forever prisoners could have their cases re-examined by the review boards.