On Sunday the body of U Khin Maung Latt was released to his family, who were reportedly told that he had died after fainting.
Photos show a bloodstained cloth around the 58-year-old's head.
Activists say he was beaten while being detained by police and soldiers, and subjected to a harsh interrogation.
Protests continue against last month's coup despite a bloody crackdown.
The UN says more than 50 people have been killed since the military detained Ms Suu Kyi, Myanmar's democratically elected leader, on 1 February.