The Airbus A320 carrying 66 people had crashed into the sea while flying from Paris to Cairo early Thursday, and some wreckage but not the black boxes has been found.
Egypt's aviation minister had said that terrorism was more likely than technical failure, but Sisi said in a televised address that "all the theories are possible".
"There is no particular theory we can affirm right now," he said.
A civil aviation official told AFP on Sunday that search teams had found none of the passengers' bodies, as the search continues.
- AFP