Japanese police have seized more than 100 smartphones stolen in a series of robberies that took place in metropolitan Tokyo and beyond, investigative sources have said.

The police investigations are making progress gradually, with more than 50 people, including alleged perpetrators, having been arrested so far, as well as action taken against those who allegedly recruited the suspects.

But the alleged ringleaders have are yet to be identified, due in part to high levels of anonymity related to the cases. Police are rushing to analyze data contained in the seized smartphones, hoping for clues to the identities of those in charge.

In Tokyo and the neighboring prefectures of Chiba, Kanagawa and Saitama, there have been 18 related robberies since late August. At least 26 cases across the country have suspected links to the robbery cases, which include trespassing and theft. These span of cases includes both Hokkaido and Yamaguchi prefectures.

The joint investigation involving police from Tokyo, Chiba, Saitama and Kanagawa suspects that so-called tokuryū — ad hoc anonymous criminal groups — are involved in the robberies.

Many of the arrested suspects have said in police questioning that they applied for yami baito ("dark" illegal part-time jobs) through social media.

However, there has been little information about the alleged ringleaders so far, and the overall picture of the tokuryū groups is unclear because of their members' anonymity and loose relationships.

Leaders of tokuryū groups often recruit perpetrators from X, formerly Twitter, and then have them use heavily encrypted messaging apps such as Telegram and Signal to communicate anonymously via smartphone. Perpetrators who do not know each other gather at a location near their target houses and run away separately after the crime.

Although police are investigating the confiscated smartphones, a certain amount of time will be needed for analyzing data in the phones partly because messages on Signal and other apps are automatically deleted over time.

In addition, the number of people involved in the crimes — including those who serve as drivers or secure weapons — make it difficult to identify the chain of command.

"Suspects involved in multiple cases are seen to be relatively close to the ringleaders," a senior investigator said, pledging to discover the masterminds who have "hidden in the internet society" partly by tracking the flows of stolen money.