The killings took place in Tardih forest of Rohtas district, about 150 km from Patna.
Additional Inspector General (Operations) SK Bharadwaj blamed the killings on the outlawed Communist Party of India-Maoist.
"All the victims were from Barachatti and Mohanpur in Gaya district," a Maoist stronghold, a police source said.
The victims were identified as members of the Sashastra People's Morcha (SPM), a counter-insurgency front made up of former Maoists and mostly active in areas bordering Jharkhand.
Police are said to have created the SPM on the lines of Salwa Judum, a counter-insurgency force in Chhattisgarh.