Lembembe was found dead in Yaounde, Cameroon, July 15, soon after he wrote about attacks in the country against organizations supportive of homosexuals.
The U.N. Office of the Commissioner for Human Rights said his body showed mistreatment possibly amounting to torture.
"This case is taking place amid a general climate of fear among Cameroonian human rights defenders," U.N. Commissioner Rupert Colville said in a statement, adding that activists in the socially conservative country have received death threats for defending the rights of the gay, lesbian, bisexual and transgender population.
Notable deaths of 2014 [PHOTOS]
White House intruder had a knife