Egyptian authorities arrested on Saturday Ahmed Tharwat Abdel-Hamid, son-in-law of Muslim Brotherhood leading member Khairat El-Shater, after his flight touched down from Qatar. Doha's ties with Egypt have been highly strained for over a year.
Cairo International Airport authorities immediately moved Abdel-Hamid to the prosecution headquarters, who had previously ordered his arrest.
The prosecution questioned Abdel-Hamid, leveling several charges against him, including membership of a terrorist organisation that aims to violate civili and personal freedoms, harm national unity, and use terrorism as a means to execute its aims.
The Brotherhood, from which ousted Islamist president Mohamed Morsi hails, was declared a terrorist organisation by the Egyptian cabinet last December. Qatar has repeatedly voiced support for the Islamist group, straining its relations with Cairo.
Investigations are still ongoing, according to a prosecution statement.
El-Shater, the Brotherhood's deputy supreme guide and a multimillionaire businessman, is currently in detention and facing trial on a variety of charges.
El-Shater's son, Saad El-Shater, was arrested 28 August 2013 in an apartment in eastern Cairo's Nasr City district and referred to the national security prosecution on charges including spreading false news about the internal situation of the country and disturbing national peace and security.
He is still being detained pending trial.
Another son-in-law of El-Shater's, Ayman Abdel-Ghani, is accused of forming an armed group called Kataeb Helwan, or the Helwan Brigades.
The allegedly self-proclaimed militant group previously released a video published and circulated on social media networks in which they vowed to target all police facilities in south Cairo.
Abdel-Ghani is currently at large.
Since the ouster of Morsi 3 July 2013, interim authorities have cracked down on Islamists, mainly the Muslim Brotherhood, detaining hundreds of the group's leaders, including El-Shater.
Charges against the Islamists mostly include inciting violence and murder and joining an outlawed group.