The theft of more than £1 million of jewellery from the Cannes film festival was "almost certainly" an inside job, police believe.
The items belonged to Chopard, the Swiss jeweller and festival sponsor which lends its collection to stars attending the event.
In the biggest theft in the festival's history, the jewellery was taken from the hotel room of an American woman working for the company, in the early hours of Friday morning.
Police say the thieves ripped a safe containing the jewels from a wall of her second-floor room at the Novotel hotel.
Detectives are continuing to question the woman, who was said to have been enjoying an evening on the town at the time. The burglary took place at some time before 5am, suggesting that she did not return until then.
"The concern is that somebody told the criminal that this employee had a lot of very valuable jewellery in her room," said an investigating source. "The cupboard safes in the Novotel are not insured - management advise that anything really valuable is kept in their main safe. It is certainly very suspicious that the thief went straight to the right room, and had the time to calmly remove the safe, and then make their escape unseen. We are almost certainly dealing with an inside job."
An official police spokesman added: "There must have been either inside complicity or people who knew that the person had jewels."
Chopard has 40 staff at Cannes, whose job is to persuade stars to wear their products. The company makes 15 per cent of its annual sales during the film festival - a figure which highlights how effective the publicity is.
Those seen wearing Chopard jewellery at this year's festival include Julianne Moore, the actress, and models Cindy Crawford and Cara Delevingne.
Chopard has provided the Palme d'Or trophy awarded to the director of the best feature film at Cannes for the past 15 years. It features a 24-carat gold palm attached to a piece of cut crystal.
Jean-Michel Cailliau, the prosecutor in nearby Grasse, is leading the official inquiry, and has valued the haul as being worth at least £1 million, more than was previously thought.
His involvement has all but ruled out theories that it could be a publicity stunt - the crime is being taken extremely seriously by investigators.
Despite this, Chopard was trying to play it down yesterday, insisting that the jewellery was worth a lot less, and was not intended to be worn by any stars.
"We can only let you know that the value of the pieces stolen is far lower than the figures announced in the media," insisted a spokesman. "The jewels stolen are not part of the collection of jewels that are worn by actresses during the Cannes film festival."
There was further drama in Cannes on Friday night when a man carrying a gun, dummy grenade and knife fired twice in the air close to an open-air television studio. Nobody was hurt and the 42-year-old was arrested.
PETER ALLEN AND HANNAH FURNESS
Copyright 2013 Telegraph Media Group LimitedAll Rights Reserved