Four people have been arrested in conjunction with Antwerp’s biggest-ever diamond robbery, which authorities now value at over $100 million.

Police say the quartet—three Italian men and a Dutch woman who was married to one of the men—spent two years planning the heist. They rented an office in the Antwerp Diamond Centre building and used their knowledge of the area to clean out 123 of its 160 safes.

The thieves apparently were in the vault area for several hours and nabbed so much that they dropped gems on the floor. Police caught them a week following the theft after recovering some discarded bounty on the side of the road.

Antwerp’s judicial director Eric Sack told the Associated Press the theft was “a piece of genius in its simplicity, not in the least because the security system was so thoroughly analyzed.” Prosecutors later intimated the thieves have connections to the Italian Mafia.

Dealers in Antwerp are still reeling from the headline-making theft and expect the process of claiming valuables to be protracted and messy. As of press time, none of the loot had been recovered.