(CNN) -- The suspected leader of an armed rebel group accused of committing mass rapes this summer in Congo was arrested Tuesday morning, a spokesman for the United Nations' stabilization mission to the African nation told CNN.
The suspect, identified as Colonel Mayele, was arrested by U.N. and Congolese forces and is an alleged leader of the militia group Mai Mai Cheka, said Leocadio Salmeron, spokesman for the Organization Stabilization Mission in the Democratic Republic of the Congo. Mayele was transferred to the army's military justice section in Goma, in eastern Congo, Salmeron said.
An investigation will get under way soon, the spokesman said, adding that he did not know whether Mayele had been charged.
Last month, a preliminary U.N. report slammed security forces for failing prevent the wave of mass rapes that occurred over several days during the summer.
The preliminary report confirmed that at least 303 civilians were raped between July 30 and August 2 in the Walikale region of North Kivu province.
The report pointed to serious shortcomings in the preparedness and response of the local detachments of the Congolese army and the police stationed in the area. It also noted that their failure to prevent or stop the attacks was compounded by subsequent failings on the part of stabilization mission forces.
The report said the forces had not received any specific training in the protection of civilians, and suffered from a number of operational constraints, including their limited capacity to gather information, as well as the lack of a telecommunications system in the area.
"The scale and viciousness of these mass rapes defy belief," said U.N. High Commissioner for Human Rights Navi Pillay. "Even in the eastern part of DRC where rape has been a perennial and massive problem for the past 15 years, this incident stands out because of the extraordinarily cold-blooded and systematic way in which it appears to have been planned and executed."
The known victims included 235 women, 52 girls, 13 men and 3 boys, and many more are believed to have been assaulted. At least 923 houses and 42 shops were looted and 116 people were abducted in order to carry out forced labor.
The report said the area had become especially unstable, and that rebels "systematically" took action locals regarded as pro-government.
Investigators were told by victims and local leaders that the main motive for the attacks "was to punish and subjugate the local population whom the attackers viewed as 'traitors.'"
Around 200 members of three armed groups carried out the attacks, the report said. They are Mai Mai Cheka, the Democratic Forces for the Liberation of Rwanda (FDLR), and elements close to Colonel Emmanuel Nsengiyumva, an army deserter who has also in the past been involved with a rebel group.
The attackers -- armed with AK47s, grenades and machetes -- launched the attacks after they said they had come to provide security for the population. The report said that while one group had been looting and raping in a village, another would ambush fleeing people, who were raped or taken into forced labor.
CNN's Salma Abdelaziz contributed to this report