Geneina had been a top campaign aide to Sami Anan, whom the military also detained after he announced he would stand in last month's presidential election against incumbent Abdel Fattah al-Sisi, who won overwhelmingly.
Geneina had said the documents Anan allegedly possessed "revolve around political events and crises Egyptian society has passed through" since the January 2011 uprising which toppled veteran strongman Hosni Mubarak.
He said they were held at a "secure" place abroad and could be released if Anan is harmed.
Moataz Wadnan, the journalist who conducted the interview, has also been arrested.
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