Takhir Mukhamedzyanov, 51, whose body was found on June 5, had fought for the rights of coal miners for many years.
The cause of death was not immediately available.
Reports suggested there were no signs of violence on Mukhamedzyanov's body.
An official autopsy report is expected later this week.
Colleagues told RFE/RL that Mukhamedzyanov had received numerous threats to his well-being by phone, his garage and car were blown up last year, and papers connected to his research were stolen.
Mukhamedzyanov had recently been involved in working with activists defending the rights of oil workers in the town of Zhanaozen, where at least 16 protesters were shot dead by police in December.