PARIS -- Today, CBS News learned the Airbus A320 transmitted messages that smoke was detected on board before it crashed in the Mediterranean, killing 66 people.

There's been no evidence the Egypt Air crash had anything to do with security at Paris' Charles de Gaulle airport, but the French have tightened it anyway. And they've launched an investigation into whether there was a lapse here that might have allowed an explosive device to be placed on board -- if one was. This isn't the first time they've been worried.

The 5,000 union security workers at the airport were all subjected to renewed, rigorous police checks following the Paris terror attacks last year. Some had their security clearance badges revoked.

The airport sits in Paris' northern suburbs, which have a high percentage of immigrants from Muslim countries, many of whom find work here.

The new security checks look for evidence of radicalization: Where the employees have travelled, what they read, who they associate with. It's known as the 'insider' problem and is acknowledged -- here and elsewhere -- in the aviation world

"Clearly some countries are more vulnerable than others to this," said Mike Vivian, the former head of operations for Britain's Civil Aviation Authority."And it can be surreptitious, below the radar, so to speak. It is a serious issue, it is being looked at."

The fact that the Egypt air plane had been to Eritrea and Tunisia -- both with their own security problems -- just before it came to Paris, adds to the concerns. Aviation security is an international problem and is only as strong as its weakest link.