U.N. regional chief Hiroute Guebre Sellassie says the Mai Mai group claimed Sadoke Kokunda Mayele had tarnished their name and wanted to get rid of him.
Mayele was an officer in the Congolese army before he joined the militia led by Commander Cheka.
A nurse who treated rape victims told the AP that one of Cheka's wives and other family members were among those assaulted.
Sellassie said Wednesday she hopes for more arrests in the attacks which lasted four days in an area within 20 kilometers (12 miles) of a U.N. peacekeepers' camp.