Sayed Ali Geneidy, 63, died at the Demu prison in Fayoum on Thursday. He was detained pending investigations into the storming of a police station after the dispersal of the Brotherhood sit-ins in August 2013.
Preliminary examinations by the prison's doctor suggested the cause of death was heart disease.
The Forensic Medicine Department was called in to determine the exact cause of death.
Edited translation from Al-Masry al-Youm
