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.

The suspects, 11 men and four women aged between 20 and 69, are all 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.

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Sam Sin Kwok-ming, superintendent of police (operations) for the Crime Wing at the Hong Kong Island Regional Headquarters, said two staff members of a Japanese company dealing in virtual currency and luxury goods had placed 1 billion yen in four suitcases on the day of the incident.

While on their way to a currency exchange 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 had a well-laid plan for the robbery. “They leaped out of the private car when the two staff arrived and snatched the four suitcases filled with cash. The entire process was done less than 30 seconds,” said Man Chi Yeung, chief inspector of Police Force, CIB 1, Hong Kong Island Region.

The four suitcases were later transferred to another seven-seater vehicle nearby, and the original private car was abandoned near the crime scene. A 20-centimeter-long knife, a mask, and some drinks were found in the first car. The second getaway vehicle was found abandoned in the New Territories.

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, some and currency were found on him, so further investigation is still needed, police added.

Valeria Shan contributed to the story.