According to the Miami Herald, the US Defense Department said Rishikof and the legal adviser for military commissioners Gary Brown were both removed from their jobs.
Mattis designated the general counsel at the Defense Logistics Agency, Jim Coyne, to serve as acting convening authority, the report said.
There is no word on why Mattis decided to dismiss Rishikof, but media reports suggest it may be due to several recent controversial decisions, including the rejection of charges for three former CIA captives and suspending the contempt sentence for the chief defense counsel.
Despite his predecessor's aspirations to shut down the camp, surrounded by controversy over arbitrary detention grounds, torture allegations and lack of access to proper judicial process for detainees, US President Donald Trump announced in his State of the Union address recently, that he had signed an order to keep the detention center at Guantanamo Bay, Cuba open indefinitely.