Paulo Malhaes was suffocated by three men who broke into his house and stole two computers and some antique guns. Last month, he gave Brazil's National Truth Commission an account of how he participated in the abduction, torture and killing of political prisoners.
Police are investigating whether the crime resulted from a robbery or an act of political revenge. But members of the commission already fear Mr Malhães' death will dissuade other witnesses from coming forward and cow others into silence.
"Some people are afraid now," said Rosa Cardoso, a lawyer who is one of the commission's leaders. "It will make our work more challenging, at least for a time."
The commission was created in 2011 by President Dilma Rousseff, herself a former leftist guerrilla who was jailed and tortured by the military during the 1970s.
The group's mission is to uncover new information regarding the deaths of more than 300 Brazilians during the dictatorship, and thousands more who were tortured. The commission cannot criminally prosecute anyone based on its findings, due to a 1979 law that the military passed while still in power giving its members amnesty for such crimes.