Cannes (CNN) -- Investigators were at work Tuesday looking into the weekend theft of jewels worth close to $136 million from an exhibition in the French resort city of Cannes, the prosecutor's office in nearby Grasse said.
A reformed jewel thief told CNN that the robbery Sunday morning at a jewelry exhibition in the luxury Carlton Hotel -- the third gem theft around the city since May -- had all the hallmarks of a professional job.
The robber, whose face was covered by a hat and a scarf, had a semiautomatic pistol and threatened to shoot the exhibitors and guests, according to Philippe Vique, deputy prosecutor for organized crime in the prosecutor's office.
Security guards were present but unarmed, he said. No one was injured.
"It was definitely a professional heist, I've been saying that from day one," said Larry Lawton, who himself served time for jewelry thefts but is now an author and motivational speaker.
He suspects the Cannes thief had an "inside guy" who provided information on where to find the jewels.
"It's definitely not a lone man, it was too coordinated," he said. "They went in and within five minutes they knew exactly where to go."
Investigators will likely be studying security footage to identify who may have been "casing the place" in the days before the theft, he said. They also will try to identify the getaway vehicle, he said.
Lawton said he was surprised the jewels were not fitted with an electronic security device that would have given anyone carrying them beyond a certain distance an electric shock. But he urged against blaming the security guards for the theft.
The gems may well already be in the hands of unscrupulous buyers, who probably paid about a fifth of what they're worth, he said.
The CGT union, representing workers at the hotel, criticized the management of the Carlton for what it called "irresponsibility."
Despite the growing number of events showcasing luxury goods, and the resulting insecurity, "the Carlton management has favored financial operations over the safety of personnel," a CGT statement said.
The union called on the French government to initiate an investigation into the safety of jewelry exhibitions in hotels.
The Nice prosecutor's office told CNN on Monday that the value of the stolen jewels was close to $136 million, more than twice what had previously been stated.
The jewels were supposed to have been on display until the end of August but signs advertising the Leviev exhibition have now been taken down from the hotel.
A spokesman for Carlton Hotels in Paris said Tuesday that the company had "no further information or statement" to give on the incident.
The Leviev jewelry firm has flagship boutiques in London, New York, Singapore and Dubai. It boasts of being one of the largest diamond manufacturers in the world, allowing it to offer "the industry's most extraordinary assortment of large, rare and colored diamonds."
The robbery follows two major jewel heists this year during the Cannes Film Festival in May.
A necklace worth $2.6 million was stolen from a hotel party shortly after jewels worth more than $1 million were stolen from a safe in a hotel room.
Jeweler de Grisogono said the necklace was part of a collection making its debut. The jewels stolen from a safe were by the Swiss firm Chopard, which was sponsoring the festival.
CNN's Alexander Felton reported from Cannes and Laura Smith-Spark wrote in London. CNN's Saskya Vandoorne contributed to this report.