Hong Kong police have arrested 15 people in connection with a daring 1 billion yen ($6.4 million) robbery in Sheung Wan on Thursday.

Seven of the arrestees were charged with conspiracy to commit the robbery that police described as “well planned”, according to a news conference on Monday morning.

All the suspects, 11 men and four women aged between 20 and 69, are local residents and are believed to include the heist’s mastermind and accomplices tasked with acting as lookouts and getaway drivers.

Some are suspected of engaging in organized crime, said police, who are still striving to retrieve the stolen money.

Police superintendent Sam Sin Kwok-ming said the two victims were employees of a Japanese company dealing in virtual currency and luxury goods. They had brought 1 billion yen in cash to Hong Kong for business reasons and planned to exchange it for virtual currency and Hong Kong dollars at a currency exchange shop on Wing Lok Street. They had conducted transactions with the shop before without incident.

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On the day of the robbery, the two victims placed the cash in four suitcases. While on their way to the dealer, a private car suddenly pulled up and three robbers sprang out and threatened the two men. After grabbing the suitcases, the robbers jumped back into the car and fled.

The police said that the gang was highly organized and violent, having lain in wait near the ambush scene. “They leaped out of the private car as soon as the two victims appeared and snatched the four cash-filled suitcases. The entire attack was over in less than 30 seconds,” said Man Chi-yeung, chief inspector of Police Force.

Later, two other thieves helped transfer the four suitcases to another seven-seater vehicle nearby, and the original private car was abandoned near the crime scene. A 20-centimeter-long knife, some used masks and empty beverage containers were confiscated in the first car. The second getaway vehicle was found abandoned in the New Territories.

During the investigation, police reviewed a substantial number of footages of closed-circuit television to identify the suspects. They also seized a large amount of evidence at the crime scene and the location where the suspects were captured, including the clothes the criminal was wearing at the time of the crime.

The criminals had clear and detailed divisions of labor, such as casing the place, committing the crime on the spot, and fleeing, police said.

As for the reason why the two employees were targeted, the police said they are still investigating.

A 43-year-old man arrested on the day of the incident outside Shun Tak Centre in Sheung Wan is not closely related to the robbery. However, he was carrying some cash and restricted drugs, so further investigation is still needed, police added.

Valeria Shan contributed to the story.