A submarine belonging to Egypt's Oil Ministry that could operate at a depth of 3,000 metres under the sea level had been deployed for the purpose, Sisi said in a live televised address, also his first public comments on Thursday's crash.
"This (submarine) moved today in the direction of the plane crash because we are working hard to retrieve the two boxes, which are the black boxes," he said, in remarks that began with a minute of silence, adding: "All the theories are possible."
Investigators continued to piece together clues for a potential breakthrough even as some reports of an audio from the jet and even of locating the black boxes emerged.
Multinational searchers scouring the waters 290 kms north of Alexandria have made headway by recovering debris, passengers' belongings, body parts, luggage and aircraft seats from the jet, that initially went 'missing' and was later declared crashed.
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His remarks come a day after French authorities also said "all theories are being examined and none is favoured".
Smoke was detected inside the cabin of the Airbus A320 minutes before it plunged into the Mediterranean Sea with 66 people on board, including children.
France's aviation safety agency said the plane had transmitted automated messages indicating smoke in the cabin and the pilot's flight control unit.
Control: "EgyptAir804 contact Padova 1-2-0, decimal 7-2-5, good night."
Pilot: "This is 0-7-2-5 Padova control. (Unintelligible) 8-0-4. Thank you so much. Good day, er, good night."
Egypt's military displayed wreckage and personal
belongings yesterday. The chunks of debris included an uninflated life vest, a seat, a purse, shoes, carpet, a scarf, parts of chairs and cushions and a sling bag. The EgyptAir label appeared on one piece of wreckage.
Egyptian Foreign Minister Sameh Shoukry said "the search is ongoing".
Shoukry said it was not clear how long the searchers will take to recover the cockpit voice and flight data recorders - the so-called black boxes - to shed crucial information about what was going on during the final moments before the crash.
"We do not, I think, have the technical abilities to operate in such deep waters, whereas many of our partners might have this facility," he said, referring to the US, France, Britain, Russia and others cooperating in the search.
French authorities have said that "finding the plane is of course the priority, along with finding the black boxes to analyse them, which will allow us to answer legitimate questions".
France's "dual goal" is to offer "solidarity with the families but also transparency... On the circumstances of this plane's disappearance," said Foreign Minister Jean-Marc Ayrault, at yesterday's news conference with the families of the victims, that include 30 Egyptians, 15 French, two Iraqis, and one person each from the UK, Belgium, Kuwait, Saudi Arabia, Sudan, Chad, Portugal, Algeria and Canada.