A $1.3 million reward was offered Tuesday after diamonds and jewels were stolen in what is believed to be the world's biggest heist.

A masked gunman made off with an estimated $136 million worth of gems from the Carlton Intercontinental Hotel in Cannes, France, during the audacious daytime raid on July 28.

But despite a massive manhunt, French authorities have not made any arrests. SW Associates, a Paris-based loss adjuster and risk manager working for insurance firm Lloyd's of London, is hoping a financial incentive might bring clues.

"A reward of up to 1,000,000 euros ($1.3 million) pro rata is offered to the first person who provides information which leads to recovery of the goods," SW Associates said in a statement.

SW Associates managing director John Shaw told Reuters he hoped the reward was enough to encourage a witness, a person close to the robbers or someone who has been approached to buy the jewels to come forward with information.

"We have to get the jewelry back first, evaluate the value of the recovered jewelry and then, once the police are satisfied we're not about to recompense the robbers, we can arrange the payment," he added.

French police are also continuing their hunt for the lone robber who entered through a French door at the side of the hotel.

It was not broken, so investigators are unsure if it was left open or whether the thief had cracked it open.

He then made his way to the temporary jewelry exhibit held over the summer by the prestigious Leviev diamond house, owned by Israeli billionaire Lev Leviev.

Armed with a handgun he threatened three private security guards, two vendors and a manager of the sale, before shoveling the jewels into a briefcase.

He then left with 72 pieces including rings, pendants and diamond-encrusted earings, although he dropped some of the jewelry as he made his escape.

No customers were present during the crime.

Leaving through a different French door he jumped about three feet onto the terrace next to the hotel before darting into a side street.

The incident was over in a minute.

The heist came two months after two smaller jewelry robberies hit the annual Cannes Film Festival, where many of the world's top movie stars are lent gowns and gems to parade on the red carpets and at glamorous parties.

The crime followed recent jail escapes by members of the so-called Pink Panther gang.

It's not the first time SW Associates has offered a huge reward for information.

In 2008, the firm offered a $1 million reward for information about a $105-million jewel heist from the Harry Winston store in Paris and recovered more than half the stolen pieces after a person came forward.