Wearing a scarf to mask his face, the gunman sneaked into the posh Cannes hotel and held up a diamond show as three security guards looked on, then fled on foot about a minute later. In the end, he made off with a breathtaking $136 million worth of valuables — the biggest jewelry heist in years, maybe ever.

It was a French Riviera robbery that might make Hollywood scriptwriters smile. And it even happened at a hotel that was featured in Alfred Hitchcock’s jewel-encrusted thriller “To Catch a Thief.”

On Monday, a state prosecutor provided new details about the brazen heist a day earlier at the Carlton Intercontinental hotel — not least that the loot was actually worth more than twice the €40 million ($53 million) estimate that police had first announced.

The noontime caper Sunday along the town’s seaside promenade, La Croisette — a playground for the rich and famous, tourists, and most notably, world cinema stars every year — looked set to dwarf the value of two other jewelry thefts in the Riviera during the Cannes Film Festival in May.

It also could eclipse two other massive heists over the last decade. In 2008, thieves — some dressed as women — stole $118 million in rings, necklaces and luxury watches from the Harry Winston store in Paris. A robbery five years earlier at Belgium’s Antwerp Diamond Center netted an estimated $100 million.

Philippe Vique, assistant prosecutor in the nearby town of Grasse, said the show’s Dubai-based organizer — whom he would not identify — had carried out a more complete inventory of the jewelry by Monday, and came up with the $136 million figure.

In a telephone interview with The Associated Press, Vique described a canny, quick and logistically simple break-in.

So far, the suspect is believed to have acted alone. He wore a scarf, cap and gloves, and somehow got into the ground-floor showroom through the hotel’s French doors, which open out onto a terrace on the Croisette — not the main entrance.

The suspect then held up the show participants with a handgun, took the valuables, and fled through a side door onto a perpendicular street.

“He took a bag containing a briefcase and a small box,” Vique said. Rings, earrings and pendants were inside the bag. “He left on foot … it was very fast.”

As the suspect exited through the side door in his getaway, a few jewels spilled out of the bag and were quickly recovered.

The holdup itself took about a minute — with three private security guards, two vendors, and a show manager on hand, Vique said. No customers were present at the time, and no police were deployed at the hotel.

“This was a private event — so it had private security,” the prosecutor said. It was not immediately clear whether the security guards were armed.

“I wouldn’t say it was easily done — opening a locked door,” Vique said, wondering: “Why was he able to open this door?”

The jewelry was part of a summertime display centering on the prestigious Leviev diamond house, owned by Israeli billionaire Lev Leviev. It was to run until the end of August.

The show was hardly low-key. Large banners adorned with pictures of diamond rings or necklaces over Leviev’s rose-themed background were draped over the street-front terrace on the Croisette.

A Leviev spokesman declined to comment. A day earlier, the company issued a statement saying its officials were cooperating with authorities and were relieved that no one was injured in the robbery.

The Carlton, in a statement, confirmed the robbery had happened and said none of its employees or guests “were involved in or affected by the incident.” The hotel said it was cooperating with police and would not comment further on the criminal investigation.

Vique said French authorities were pursuing all possible leads — which he would not describe — and reviewing surveillance video footage, notably from cameras put in place by Cannes municipal authorities. But he said there was no indication so far that the suspect had links to any organized crime group.

Scott Andrew Selby, co-author of “Flawless: Inside the Largest Diamond Heist in History,” about the 2003 Antwerp heist, said initial estimates of jewel heists are often wrong: He believes that the safe-deposit box theft in Belgium actually brought in $400 million to $500 million.

Selby said he doubted the Cannes culprit was working alone, because the quick-in, quick-out operation seemed professional, “and people who know what they’re doing operate in teams.” He said Cannes was particularly tempting to high-end jewel thieves who are looking for a “soft target.”

“Cannes is an amazing place to rob because you have this stuff coming in just for temporary sales,” he said.

The Carlton, a Cannes fixture which is celebrating its centennial this year, featured in Hitchcock’s “To Catch a Thief,” which starred Grace Kelly as an heiress and Cary Grant as a reformed burglar chasing a jewel thief.

Fiction aside, it isn’t the first time the hotel has dealt with jewelry theft: In 1994, machine-gun-toting thieves stole $45 million in gems there.

(AP)