ROISSY-CHARLES DE GAULLE AIRPORT, France (AFP) - French investigators are checking again for security flaws at Paris Charles de Gaulle airport after Thursday's EgyptAir crash, despite already stripping hundreds of employees of their security clearance since last year's militant attacks.

It is still far from clear whether the EgyptAir plane that crashed en route from Paris to Cairo was the victim of a terrorist attack or that any security lapse happened on French soil.

But security staff are taking no chances.

Following the attacks on Charlie Hebdo magazine and a Jewish supermarket in January 2015, the airport north of Paris imposed a zero-tolerance approach to staff, stripping more than 600 of its 86,000 employees of their security clearance.

Anyone with a criminal record lost their security pass, but the police authority that handled the review said 85 were suspected of having sympathies with radical Islamists.

Another 400 members of staff are still under investigation after being flagged as possible radicalisation cases, added a source close to the investigation.

With even one radicalised staff member with access to sensitive areas posing a potentially major threat, investigators are hunting for the slightest sign of radicalisation.

The refusal of a male employee to speak to or take orders from a woman, for example, is enough of a red flag to justify pulling his badge.

Investigators stress that they are not jumping to conclusions as to what caused Thursday's disaster in which all 66 passengers and crew were killed.

But they have questioned any ground staff who had the least contact or involvement with the plane.

The investigation requires close collaboration between the different services, including the air transport police (the GTA); the border police (the PAF) and the intelligence services.

They are striving to build a clear picture of what happened during the doomed plane's brief passage through Paris.

The Airbus A320 arrived in Paris from Cairo at 9.55pm Wednesday (1955 GMT). It took off again a little more than an hour later, at 11.09pm.

Investigators are using everything at their disposal, including video surveillance, to identify passengers, crew, maintenance staff and baggage handlers who had contact with the plane.

But so far they have not found any suspect or any flaw in the system, said a source close to the investigation.

Another source close to the probe confirmed that the security measures in place seemed to have functioned correctly.

"The terrorist hypothesis has not been ruled out, but more in the sense of a human action than a bomb," the source told AFP, implying that if there was foul play, it would have been caused by someone on the plane rather than a planted device.

One source at the airport said no freight had been loaded on the plane at Paris and the luggage in both the hold and cabin had been thoroughly checked.

It seemed "highly improbable" then that an explosive device was smuggled on board at Paris.

Investigators are also looking at the passenger list.

"It's much too soon to say whether or not there is an individual with a problematic profile," said one of the investigation sources.

"But that doesn't mean there are not suspicions."

Security in all French airports has been tightened since last November's attacks, air security specialist Xavier Tytelman told AFP.

"After a first check with scanners, 30 percent of the luggage considered suspect is taken to a second system, special scanners for detecting explosives."

A final check was an inspection by hand with demining teams standing by, he added.

Hand luggage, meanwhile, is checked by scanners that are "99.9 per cent reliable", Tytelman said.

"You would be hard-put to do more."

Profiling has also been introduced in Paris since the March 22 bombings at Zaventem airport in Brussels that killed 16 people.

The job of profilers is to observe the tens of thousands of people passing through the terminals on the look-out for anything that strikes them as abnormal behaviour.