The 22-year-old daughter told police she was kidnapped from her apartment in the southeastern French city of Lyon by two men posing as plumbers.
She said the men bundled her into a car, tied her up and forced her to telephone her father who works for SOS Surveillance, a Swiss company that transports cash in armoured vans, to demand a ransom.
The father, who was en route with a colleague to the Swiss city of Lausanne with a cash delivery, then agreed to meet the kidnappers in a car park near the border with France and hand over the van's contents, the story went.
French police last week estimated the loot at 20-30 million Swiss francs (€17–26 million).
The daughter was found unharmed at around 10:00 pm on a road outside Lyon and appeared "very shocked" by her ordeal, police said at the time.
Investigators now appear to suspect it was an inside job.
Swiss authorities had appealed for witnesses to come forward over the robbery and said they were looking for three men with accented French, "maybe from the south of France or North Africa".