Egypt's president said on Sunday a submarine belonging to his country's Oil Ministry was headed to the site of the crash of EgyptAir Flight-804 in the eastern Mediterranean to join the search for the cockpit voice and flight data recorders, commonly known as black boxes.

President Abdel-Fattah el-Sissi also said Egypt was jointly investigating the Thursday crash with the French government. "It is very, very important to us to establish the circumstances that led to the crash of that aircraft," he said in comments broadcast live on Egyptian TV channels.

He said the submarine, which has the capacity to operate at a depth of 3,000 meters below the surface, left for the site Sunday. He gave no further details.

Making his first public comments since the crash of the Airbus A320 while en route from Paris to Cairo, el-Sissi says it "will take time" to determine the exact cause of the crash, which killed all 66 people on board.

The announcement comes just one day after the first available audio from the doomed EgyptAir Flight 804 was released.

The audio showed the pilot in normal contact with Swiss air traffic controllers as the flight continued on its course from Paris toward Cairo, hours before officials lost contact with the jet.

The seemingly standard dialogue came to light hours after Egypt denied media reports the doomed jet's black boxes had been located.

The new audio indicated that all was routine as the plane checked in with air traffic controllers in Zurich late Wednesday night, before being handed over to Italian air traffic controllers in Padua.

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."

The audio recording was taken from www.liveatc.net, a website that provides live air traffic control broadcasts from around the world.

The communication occurred around midnight local time, about 2 ½ hours before Greek air traffic controllers in Athens lost contact with EgyptAir Flight 804.

A senior official at the Egyptian Civil Aviation Ministry refuted the reports as did a spokesman for Egypt’s military who said he had no information to share on the retrieval of the black boxes, or cockpit voice and flight data recorders.

The statements came as French investigators announced Saturday that smoke was detected in multiple places on the ill-fated EgyptAir plane before it plummeted into the Mediterranean Sea.

The Aviation Herald initially reported that sensors detected smoke in a lavatory, suggesting a fire onboard the aircraft before it went down.

David Learmount, a noted aviation expert, told Fox News Friday that the data received from some satellites indicates that a fire could’ve started in the avionics compartment of the plane which knocked out computers and control mechanisms on the flight. He said that would’ve caused the plane to crash.

French officials didn’t say what could have caused parts of Flight 804 to fill up with smoke. Spokesman Sebastien Barthe told the Associated Press the plane’s automatic detection system sent messages indicating smoke a few minutes before it disappeared from radar.

He said the messages “generally mean the start of a fire.”

"We are drawing no conclusions from this. Everything else is pure conjecture,” Barthe added.

According to The Wall Street Journal, sensors aboard the plane detected smoke in the lavatory on the aircraft’s nose. The paper, citing someone familiar with the aircraft’s data, reported the messages suggest there was damage to the right-side of the cockpit windows.

Officials who have reviewed the data told the Journal that the broadcast information by itself is insufficient to determine whether the plane crashed because of a bomb or another cause. Rockwell Collins, a global aviation telecommunications provider, said late Friday it transmitted the messages over its networks to the carrier in real time.

Learmount said the possibility of terrorism couldn’t be ruled out. He said there were a couple other scenarios that could have caused the plane to crash.

A terrorist got into the avionics compartment and placed a device that triggered the fire and alarm
That all of these sensors were picking up smoke and then disablement following a bomb going off.
That the fault in the avionics and the terror attack happened concurrently.

An EgyptAir official confirmed Friday that wreckage of the missing plane was found, including body parts, luggage and passengers’ seats. The announcement came hours after a Greece official also reported evidence being found.

"A short while ago we were briefed by the Egyptian authorities... on the discovery of a body part, a seat and baggage just south of where the aircraft signal was lost," Defense Minister Panos Kammenos told reporters in Athens, according to Reuters.

The Cairo-bound Airbus 320 had left Charles de Gaulle Airport in Paris late Wednesday night with 66 on board, and disappeared from radar as it neared its destination.

A mile-long oil slick was identified from satellite images from the European Space Agency, which cautioned that there was no guarantee the slick was from the missing aircraft. The agency said the slick was about 25 miles southeast of the plane's last known location, and passed the information to relevant authorities late Thursday.

The Egyptian presidency Friday expressed its "deep sadness and extreme regret" over the deaths of the passengers and crew members aboard the flight -- the first official recognition of the tragic crash.

Egyptian army spokesman Brig. Gen. Mohammed Samir wrote on his Facebook page that Egyptian jets and naval vessels participating in the search for the missing plane had found "personal belongings of the passengers and parts of the plane debris."

No terror groups has taken credit for the disaster and authorities were going through the passenger manifest, crew members' backgrounds and airport staff for possible links to terror.

Authorities said the plane swerved and spun wildly before plummeting into the sea. The Egyptian military said that no distress call was received from the pilot.

In Paris, French authorities scoured Charles de Gaulle Airport, the country's main hub, for any sign of a security breach prior to the flight's departure. Reuters reported that investigators were interviewing officers who were on duty at the airport Wednesday night to determine whether they heard or saw anything suspicious.

"We are in the early stages here," a police source told Reuters about the investigation.

Flight 804 was carrying 56 passengers, including one child and two babies, three security staff and seven crew members, officials said. Egypt's aviation minister, Sharif Fathy, described those on board as including 15 French passengers, 30 Egyptians, one Briton, two Iraqis, one Kuwaiti, one Saudi, one Sudanese, one Chadian, one Portuguese, one Algerian and one Canadian.

In Egypt, home to 30 of the victims, grieving families and friends wondered if their loved ones would ever be recovered. Many gathered in mosques for Salat al-Ghaib, or "prayers for the absent," held for the dead whose bodies have not been found.

"This is what is ripping our hearts apart, when we think about it. When someone you love so much dies, at least you have a body to bury. But we have no body until now," said Sherif al-Metanawi, a childhood friend of the pilot, Mohammed Shoukair.

Among those killed were Salah Abu Laban, his wife Sahar Qouidar, their son Ghassan Abu Laban and daughter-in-law Reem al-Sebaei

The relative, Abdel-Rahman al-Nasry, told The Associated Press, "I ask God for forgiveness. This is very hard for the family."

Magdi Badr, a family friend, said, "we pray for the victims."

Fox News’ Greg Palkot and The Associated Press contributed to this report.