What is the fine-grained topic of the following text: The Wall Street Journal reported on Saturday on the case of a Marine Corps colonel, V. Stuart Couch, who refused to handle the prosecution of a Guantánamo detainee, Mohamedou Ould Slahi, after concluding that the evidence he was asked to use to press the case had been secured through the use of torture. Only return the topic.
Marine Corps colonel's refusal to prosecute a Guantánamo detainee due to evidence secured through torture