Jamaa Islamiya leading figure Essam Derbala died in prison on Sunday, with the Islamist group saying he was prevented from access to medicine.
The Jamaa Islamiya is a hardline Islamist group which was implicated in several militant attacks in Egypt during the 1990s, before its jailed leaders renounced violence in 1998.
The group said it holds "political and security authorities" accountable for Derbala's death, in a statement on its website.
Jamaa Islamiya said that for months, Derbala was prevented from accessing healthcare and that he went into a coma as a result. The group added that all attempts to have him hospitalised had failed, despite his daily deterioration.
However, an official from the south Cairo area prosecution, Tarek Abu Zeid, told Aswat Masriya that a prosecution representative inspected Derbala's body inside prison.
Abu Zeid said medical reports indicate he died as a result of "high blood pressure" and another "medical condition," which he long suffered from. Abu Zeid did not specify the medical condition.
Derbala was arrested in May, following an arrest warrant issued by the State Security Prosecution, a branch of the prosecution which is typically involved in cases that affect national security.
He was charged with joining the pro-Mohamed Morsi Anti-coup alliance, a coalition formed to call for the reinstatement of the former president who was ousted in July 2013.
