Search crews have found human remains, along with the personal belongings of passengers from EgyptAir Flight 804, but are still searching for the bulk of the wreckage.
Looking for clues to understand why the jet carrying 66 people crashed into the Mediterranean Sea, search teams sought to find larger pieces of the plane or the location signal from the flight recorders.
The European Space Agency said one of its orbiting spacecraft spotted what could be an oil slick 40 kilometers southeast of the plane's last known position and the information had been passed on to relevant authorities conducting a search mission.
The agency released a grainy photograph of the scene which showed little detail and cautioned there was no certainty the slick was from the aircraft.
Also Friday, Egyptian authorities say they are investigating reports that there was smoke on the flight before it crashed.
An aviation industry publication reported that sensors detected smoke in the lavatory, indicating that there could have been a fire on board the plane.
Egypt Aviation Minister Sherif Fathy has said a terrorist attack is a more likely cause for the crash than technical failure. But so far no clear evidence has emerged as to what brought the plane down. No militant group has claimed to have carried out an attack.
The plane disappeared into the Mediterranean Sea, north of the Egyptian coast, on Thursday while headed to Cairo from Paris.
The Airbus A320 had been flying normally when it suddenly swerved radically and plunged more than 11,000 meters into the sea, never issuing a distress signal.
Three French investigators and a technical expert from the A320's manufacturer, Airbus, arrived Friday in Cairo to aid in the investigation.
U.S. Secretary of State John Kerry offered Egypt continued support for the search Friday. Kerry told Egyptian Foreign Minister Sameh Shoukry the United States promised to stay in close contact as the investigation progressed.
Egypt, France, Greece and the U.S. are among nations deploying planes and naval vessels to search for the plane.
WATCH: People Pray in Cairo for EgyptAir Victims
France’s foreign minister on Friday rejected widespread comments that terrorism is to blame.
"We're looking at all possibilities, but none is being favored over the others because we have absolutely no indication on the causes (of the crash)," Jean-Marc Ayrault said on French television.
Prayers and a symbolic funeral for the passengers were held Friday at Cairo's Sultan Hussein Mosque.
Aviation experts are warning against speculation on the cause of the crash.
“I will say that when an airplane disappears at 37,000 feet it’s a highly unusual event," Scott Hamilton of Leeham Aviation Consultancy told VOA. "It either typically indicates a catastrophic failure, catastrophic emergency of some kind, or as we know from not too long ago, a bomb could go off... But you just have to be cautious and not jump to any conclusions at this point.”
Fred Burton of the U.S.-based global intelligence company Stratfor tweeted, "Mechanical failure at cruising altitude is unlikely. Such an event typically occurs at takeoff or landing."
EgyptAir sent interpreters and doctors to the Cairo airport to meet with the passengers' families.
The disappearance has renewed security concerns months after a Russian passenger plane was shot down over the Sinai Peninsula. The Russian aircraft crashed in Sinai on October 31, killing all 224 people on board.
Moscow said it was brought down by an explosive device, and a local branch of the extremist Islamic State group claimed responsibility for planting it.
Joshua Fitzick, Esha Sirai in Washington and Steve Herman in Bangkok contributed to this story; additional reporting by Victor Beattie and National Security correspondent Jeff Seldin in Washington.
