Police in Tokyo and three surrounding prefectures have launched a joint investigative team over a series of robberies and other crimes that are likely being carried out by people solicited through social media. The police believe at least 14 incidents may be linked to each other.

Senior investigators from Tokyo, Saitama, Chiba and Kanagawa on Friday attended the inaugural meeting of the task force.

Investigators have so far arrested 29 people in nine cases that have occurred since August. But the suspects are all minions, or worked as lookouts, and police have not arrested the mastermind who gave them their orders.

Police have found out that in some of the cases, the mastermind used the same account on a highly anonymous messaging app.

The head of the joint team, Shinka Kazuhito of the Metropolitan Police Department, says the perpetrators are becoming increasingly violent from incident to incident, and residents are living in great anxiety.

He also says Japanese police must do all it can to quickly identify the "bad guys" who are behind the minions, and get to the bottom of the criminal groups and the crimes they are perpetrating.

The police have been mobilizing about 300 officers who specialize not only in robberies, but also fraud, cybercrimes and organized crimes.