Japanese police have arrested four men suspected of masterminding a series of robberies in the Tokyo metropolitan area, including a case that left a woman seriously injured in Chiba Prefecture in October last year.

It is the first time that masterminds have been arrested in connection with the robbery cases in which over 50 perpetrators have been arrested.

The four men are Hiroto Fukuchi, 26, Takuya Saito, 26, Karura Murakami, 27, and Shota Watanabe, 26. Police believe that the suspects formed a ³Ù´Ç°ì³Ü°ù²â¨± anonymous and transient criminal group that was behind the robberies.

The four were arrested on suspicion of beating and kicking a 51-year-old woman at her home in the city of Ichikawa, Chiba Prefecture in the small hours of Oct. 17 last year and robbing her of 15 items, including cash and a car. The assault left her with serious injuries that took two months to heal. The woman was bound with adhesive tape and abducted to a hotel in Saitama Prefecture near the capital where she was held for about 14 hours.

In this case, three men in their 20s who carried out the crime have already been indicted for robbery resulting in injury, and three people in their 20s to 30s have been indicted for transporting stolen goods. Some of them have been sentenced to imprisonment.

The four masterminds recruited perpetrators for ¡°dark¡± part-time jobs, or yami baito, on social media, according to people familiar with the investigation. They used the highly secure communication app Signal to provide instructions, including information such as the address of the woman and the method of attack, using nine different account names, such as ¡°Patrick,¡± ¡°Falcon¡± and ¡°PTA.¡± Stolen items, including cash, ended up in their hands.

Some among the four are believed to be connected to gangs. They appear to have gotten closer because they belong to the same generation.

Since the end of August last year, there have been nearly 30 cases of robberies and break-ins committed by people recruited as perpetrators in the Tokyo area, including a case in which a 75-year-old victim, Hiroharu Goto, died in Yokohama, Kanagawa Prefecture, in October last year. So far, 51 people have been arrested.