As French authorities question airport staff who had access to EgyptAir Flight 804, cleaning crews are among those drawing attention.
One theory is that a bomb could have been placed in the plane while it was on the tarmac in Paris, or at its previous stops in Cairo or Tunis, although there is no evidence so far of a bomb being aboard the flight that crashed on Thursday into the Mediterranean while flying from Paris to Cairo.
Sylvain Prevost, who trains Paris airport personnel, said cleaning staff are trained to alert authorities to suspicious items but specialised security personnel are not routinely required to sweep a plane after the cleaning crew leaves.
He noted that rules vary from airport to airport and said he was not aware of the procedures used when the EgyptAir plane was parked in Paris' Charles de Gaulle airport.
Mr Prevost noted that despite extensive efforts to ensure security, workers can sometimes be threatened into cooperating with criminals.
Meanwhile, a senior official at the Egyptian Civil Aviation Ministry has denied media reports that EgyptAir Flight 804's cockpit voice and flight data recorders, commonly known as the black boxes, have been located.
Military spokesman Brigadier-General Mohammed Samir also said he had no information to share on the location or the retrieval of the black boxes.
The boxes are believed to be in Mediterranean waters around 180 miles north of Alexandria. The waters are 8,000 to 10,000 feet deep, and the pings from the black boxes can be detected up to a depth of 20,000 feet.
Earlier Saturday it was revealed that smoke was detected in multiple places - but the cause of the crash that killed all 66 on board remains unclear.
The French air accident investigation agency's spokesman Sebastien Barthe said the plane's automatic detection system sent messages indicating smoke a few minutes before the plane disappeared from radar while flying over the east Mediterranean early on Thursday.
The messages, he explained, ''generally mean the start of a fire,'' but added: ''We are drawing no conclusions from this. Everything else is pure conjecture.''
Looking for clues to whether terrorists may have brought down the Airbus A320, investigators have been poring over the passenger list and questioned ground crew members at Charles de Gaulle Airport in Paris, from which the plane took off.
The aircraft had been cruising normally in clear skies on a night-time flight to Cairo when it suddenly lurched left, then right, spun all the way around and plummeted 38,000 feet into the sea, never issuing a distress signal.