Mohammed Morsi's family has vowed to take legal action against Egypt's army for "kidnapping" the former president.
The army has held Morsi since it overthrew him on July 3 following mass anti-government protests.
Several countries, including the United States and Germany, have called for Morsi's release. However, Egypt's interim authorities have rejected those calls, claiming that they have kept him in a "safe place" since his overthrow earlier this month.
"We are taking local and international legal measures against [army chief] Abdel Fattah al-Sisi, the leader of the bloody military coup, and his putschist group," the president's daughter, Shaimaa Morsi told reporters.
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"We hold the leader of the military group and the whole of his putschist group responsible for the health and safety of president Morsi," she added at the family's press conference in Cairo.
The statement represents the first public communication from Morsi's family since the military overthrew the president on July 3. Morsi himself has been held incommunicado and at an undisclosed location for that entire stretch. Officials in the interim government have said the measures mean to protect Morsi.