The mystery over the MS804 disaster took a new twist after it was claimed the pilot carried out a "sudden descent" in a bid to extinguish a fire on board.
Aviation sources in Paris say that Mohamed Ali Shoukair told Egyptian air traffic controllers that he needed to attempt an emergency landing as the plane was filling up with smoke.
According to the French TV station M6 a conversation of "several minutes" took place as Mr Shoukair struggled to keep control of the aircraft.
The claims contradict the investigators' account of the crash as they say there was no radio contact from the pilot before MS804 crashed into the sea.
And EgyptAir officials have denied M6's report, insisting: "[The] claims made by the French TV station are not true. The pilot did not contact Egypt air control before the incident."
Emergency descents can be dangerous as they can cause major changes in cabin air pressure and, it was speculated, may have led to the plane plunging 37,0000 feet into the Mediterranean Sea.
It was not the only controversial theory behind the crash to be of offered on Monday, as a Turkish newspaper quoted an airline pilot as saying he saw "an unidentified object with green flashing lights" pass over Istanbul just one hour before the air disaster.
"Then it disappeared all of a sudden. We are guessing that it was a UFO," the pilot told Hurriet Daily News.
Egyptian investigators say it is too early to say for sure what caused the crash.
It comes as the first body parts of the victims of the MS804 crash were returned to Cairo last night in preparation for the grim task of identifying those who lost their lives in the disaster.
And in France, the Paris airports authority said it has no plans to tighten security further after the EgyptAir crash but airport workers expressed concern on Sunday about weak spots in the systems.
Pauline Godebout, an airline desk officer, said there were "sometimes" identity checks at entrances to terminals at Charles de Gaulle airport.
"But they don't do it all the time. I think it's a budget issue. It's expensive and there are so many doors to the airport that it's very difficult to secure them all."
An airports authority spokesman told the Telegraph that security had already been stepped up after recent terrorist attacks.
He said people entering the airport were sometimes required to produce identity documents before being allowed in, but this was not done systematically to avoid creating long queues outside.
"Spot checks of people's identities are carried out on a random basis," he said. "We have to strike a balance between security and inconveniencing travellers. The question is, do you really increase security or are you just moving the risk to another area?"
Identity checks were introduced outside Brussels terminal entrances after two suicide bombers blew themselves up in March in the check-in area, before passengers are routinely screened. However, the Paris spokesman pointed out that they "created queues of hundreds of people" that could also be a target.
"There is nothing to indicate a security flaw at Charles de Gaulle," he said.
Armed police and soldiers frequently patrolled the terminals on Sunday, but Céline, a parking attendant, expressed concern that a lack of manpower and funding left the airport vulnerable to an attack.
"For an airport, I think the level of security here is really not enough. We are responsible for identifying abandoned bags. We deal with about five alerts a week. And I think the police response is too slow. Even when they seal off the area, it's still not enough to protect us. It takes the bomb disposal squad between 30 minutes and an hour to arrive, and that's easily enough time for a bomb to go off."
Asked if she thought a bomb could be planted on a plane on the ground at the airport, she said: "I think it could happen. There are checks but they're not systematic enough."
She said she had been "really scared" since the Brussels bombings and was relieved that she was quitting her job in a week after a six-month contract. "I can't wait to go," she said.
As a huge international search continued, Egypt deployed a remote-controlled submarine to try to find the airliner's black boxes to determine whether it was technical failure or terrorism that caused the A320 to plunge into the sea on Thursday.
Families of the 66 victims were warned it might be weeks before bodies are recovered as the Egyptian government admitted that the submarine might be unable to operate at extreme depths.
US Navy aircraft have spotted more than 100 pieces of debris.
With no bodies to bury, an absentee funeral mass was held at Cairo's main cathedral for nine Coptic Christians who were on the plane. Photographs of two of the dead - Medhat Michel and Waguih Mourise - were placed on the altar as the bishop read out the victims' names.
Also among the dead is Islam Usama, a 22-year-old law student who had been on holiday in Paris with his elder brother but was returning early to Cairo to take exams. His brother, Mohammed, dropped him at the airport expecting to see him a few days later. When he heard the news of the tragedy the next morning, he rushed back to Egypt to be with his family.
"Since I came back haven't been able to think about anything and I'm not aware of anything of happening around me," he told the Telegraph.
EgyptAir Flight MS804: What we know
EgyptAir Flight MS804 vanished en route from Paris to Cairo, around 40 minutes from its destination.
On board: 56 passengers, 10 crew
Route: Paris Charles de Gaulle to Cairo
Departure: Left Paris at 11.09pm local time (10.09pm London time) on Wednesday 18 March
Disappearance: Lost contact at 2.39am local time (1.39am London time) over the Mediterranean sea, approximately 10 miles inside Egyptian airspace
Plane: Airbus A320, manufactured in 2003. This flight was the plane's fifth of the day. It had already travelled to Asmara, Eritrea and Carthage in Tunisia
Captain: 6,275 flying hours experience
Co-pilot: 2,766 flying hours experience
Passengers' nationalities: 15 French, 30 Egyptian, 1 British, 1 Belgium, 2 Iraqis, 1 Kuwaiti, 1 Saudi, 1 Sudanese, 1 Chadian, 1 Portuguese, 1 Algerian, 1 Canadian
Why did flight MS804 crash? The theories
Theories that have been put forward about the cause of the crash include:
1. Mechanical malfunction
"A major technical fault - the explosion of a motor, for instance - seems improbable," said aeronautics expert Gerard Feldzer, underlining that the A320 in question was "relatively new", having entered service in 2003. "In addition, the A320 has an excellent safety record as the best-selling, medium-range airliner in the world."
There were no reported signs of mechanical malfunction when the Airbus A320 set off.
2. Shot down
Experts have also said it is unlikely the plane was shot down, as the region is one of the most monitored in the world and it would be hard to conceal such an attack. Debris will be tested to see if there are any traces of explosives.
There have been no confirmed reports of any terror group claiming responsibility for bringing down the plane.
Egypt's aviation minister Sherif Fathy said: "If you analyse the situation properly the possibility of having a terror attack is higher than the possibility of having a technical [problem]".