Police said on Monday that 15 people have been arrested on suspicion of a conspiracy to rob following a one billion yen daytime robbery that happened within a space of 30 seconds in Sheung Wan last week. The incident on Thursday saw two staff from a Japanese virtual currency and luxury-goods company ambushed by three robbers on Wing Lok Street, one of whom was armed with a 20-centimetre knife, as the staff were on their way to a currency exchange shop carrying a total of four suitcases filled with HK$50 million worth of yen. The robbers fled in a car before abandoning it in Jervois Street and continuing with their escape in a seven-seater. The 15 suspects – locals aged between 20 and 69 – were arrested over the course of the next few days, police said. Seven have been charged so far. At a press briefing, superintendent Sin Kwok-ming said the force believed they had arrested the mastermind and those directly involved in the case. “We are still investigating the backgrounds of the culprit and robbers," he said. "Some of them have [suspected] triad backgrounds, but whether they are involved in other robbery cases, we are still investigating. “Whether the syndicate had colluded with other parties, an active investigation is still under way.” Police said they would look into how the suspects knew about the large sum of money and their connections with the victims and currency exchange shop staff, adding that more arrests might be made. They also called on witnesses to come forward. In addition to the 15 arrests, a 43-year-old man from the mainland was arrested outside Sheung Wan station on Thursday. Officers said he had been released on bail, with insufficient evidence so far linking him to the robbery.