Embarrassed officials were at a loss to explain how Jean-Pierre Treiber, 45, a double murder suspect, managed to elude detection in the box he had built himself at a workshop in the high security prison of Auxerre, Burgundy.
With its hidden human cargo, the box was loaded with dozens of others onto a lorry for delivery to the Yonne region, southeast of Paris.
During the 100-mile journey, he broke free and leapt from the lorry. The driver only realised there was a problem once he had reached his destination, when he spotted a hole in the tarpaulin covering the boxes, some of which were flattened.
Police have sealed off roads and a huge area of woodland in the hunt for Treiber using helicopters and sniffer dogs.
Treiber has been in prison awaiting trial since 2004, charged with murdering French actor Roland Giraud's daughter Geraldine, and her friend Katia Lherbier.
He is due to stand trial early next year in a case that shocked France.
Treiber is believed to have sneaked inside a cardboard box at 10.30am on Tuesday while he was left alone in a prisoner's workshop.
He had earlier told guards that he had an afternoon meeting with his parole officer – and nobody noticed his absence until later that evening, giving him seven hours to make good his escape.
Roland Giraud, 67, one of France's best-known actors, said Treiber's escape was an "avowal" that he had murdered his daughter.
The bodies of Geraldine Giraud, 36, and Katia Lherbier, 32, were both found dumped in a well on Treiber's property in Villeneuve-sur-Yonne, near Paris.
They had been strangled.
But Treiber's lawyer, Eric Dupond-Moretti, said his escape proved nothing.
"One can either say he's guilty and that this is proof of his guilt... or one can say he's desperate, that he no longer has faith in the justice system because he's pleaded his innocence for so long and hasn't been heard."
Treiber has always denied the murders despite having used the victims' credit cards to withdraw cash. When he was arrested and the first body discovered, his cryptic reaction was: "He bumped her off."
Mystery still surrounds unknown DNA that does not match Treiber's on adhesive tape used to gag the girls' mouths.
Police admitted they had a tough task finding Treiber, a former forest warden, who knows the Othe woods 20 miles north of Auxerre like the back of his hand. "We're searching the places where he used to hunt," said a policeman. "But he might well have fled further afield".
