Erotic best-seller Fifty Shades of Grey is popular reading among high-value detainees at Guantanamo, officials say.
"Rather than the Koran, the book that is requested most by the (Camp Seven detainees) is Fifty Shades of Grey," House Democrat Jim Moran said, in comments confirmed on Wednesday by his spokeswoman.
"They've read the entire series in English," said Moran, who supports moves to close the US detention facility in southeastern Cuba.
"I guess there's not much going on. These guys are going nowhere, so what the hell."
The unexpected Fifty Shades revelation reportedly came during a tour that delegation members took with the base commander, the chief medical officer, and the officer in charge of once-secret Camp Seven, the most secure part of the Guantanamo facility.
British author EL James has sold more than 70 million copies of her erotic romance trilogy of novels, which depict the use of handcuffs and blindfolds during sex.
Camp Seven is maximum security, and its high-value detainees - including five men accused in the September 11, 2001 terror attacks - are often hooded and handcuffed when transferred to other parts of the facility.
Journalists are allowed to visit Guantanamo, where detainees have access to censored reading material and movies, but Camp Seven is off limits.
"We don't discuss our high-value detainees except in the most generic terms," Lieutenant Colonel Todd Breasseale told HuffPost. "Further, we do not discuss the assertions made by members of Congress."
Moran's spokeswoman said the politician "does not care" what the high-value detainees are reading.
President Barack Obama vowed to close the facility when he took office in 2009, but the military prison created in the wake of 9/11 still holds 166 men.
Most held there have never been charged or tried, and 70 detainees were taking part in a hunger strike as of July 23.