The US embassy in Cairo has confirmed that a staff member has been held without charge for nearly three weeks.
Ahmed Aleiba, an Egyptian, was arrested by the authorities on 25 January, the embassy said in a statement on Thursday.
The authorities have not told the embassy why they are holding him.
Al-Arabiya news channel claimed Aleiba was arrested at a pro-Morsi march.
In July 2013 Al-Watan newspaper said Aleiba had arranged meetings between visiting US government officials and Muslim Brotherhood deputy leader Khairat El-Shater.