Poipynhun Majaw had been filing applications under the Right to Information (RTI) Act to check alleged corruption in public projects in Meghalaya's Jaintia Hills Autonomous District Council.
His body was found near a bridge in Khliehriat, the district headquarters of East Jaintia Hills, 120 kilometres from state capital Shillong. He was also the president of Jaintia Youth Federation.
Police said he was last seen riding a motorcycle near the East Jaintia Hills deputy commissioner's office on Monday night.
"A wrench was found next to the body. Preliminary inquest suggests the victim was hit on the head leading to his death," senior police officer AR Mawthoh said.
East Jaintia Hills has over a dozen cement plants.
Civil Society Women's Organisation president Agnes Kharshing condemned the murder. "We strongly condemn his killing and demand immediate arrest of those responsible," she said.