Interior Ministry official Maj. Gen. Hassan el-Sohagi said today that Essam Derbala died of natural causes.
A statement from Gamaa Islamiya's political party accused Egyptian authorities of "assassination." It said authorities intentionally deprived Derbala of medicine and subjected him to psychological torture.
The statement called for responsible officials to be prosecuted.
Derbala was arrested earlier this year and accused of inciting violence. He was imprisoned in a high-security facility in Cairo called Scorpion.
Gamaa Islamiya waged an armed insurgency in Egypt in the 1990s. The group later renounced violence and was a strong ally of ousted Islamist President Mohammed Morsi before and during his year in office.