A Parisian judicial police source told US Weekly the suspected perpetrators appeared cool, calm and collected as they carried out the heist.
“They acted like they had all the time in the world,” they said.
“It is quite incredible that they managed to hang about for so long without anyone calling us.”
The police insider explained three suspects arrived on bicycles at 2:18am before they were joined by another two on foot at 2:32am.
Both groups appeared to know the security code at the front gate.
"They clearly knew the security code of the main doors, and let themselves in without any problem at all," they said.
Police believe the heist took around 49 minutes and ended when the five men emerged and fled on bicycles or by foot, according to the source.