Tokyo: A senior member of yakuza was arrested for allegedly stealing Pokémon cards near Tokyo in April, a case seen as an example of Japanese organised crime groups struggling with declining membership.

Police agents who were busy dealing with thousands of yakuza members just a few years ago have noticed something new: unorganised and loosely connected groups they believe are behind a series of crimes once dominated by yakuza.

Police call them “tokuryu,” anonymous gangsters and tech-savvy young people hired for specific jobs. They often co-operate with yakuza, obscuring the boundary between them and making police investigations more difficult, experts and authorities say.

The Tokyo metropolitan police are investigating six suspects in their 20s and 30s, most of them without connections to one another, who are believed to have been hired on social media to kill, transport and burn the bodies of an older couple at a riverbank of Nasu, 200 kilometres north-east of Tokyo.

“It’s a crime carried out like a part-time job,” Taihei Ogawa, a former police investigator and crime analyst, said on an online talk show. “Tasks are divided, making it difficult for police to track down where instructions come from.”

The yakuza membership shrunk to 20,400 last year, a third of what it was two decades ago, according to the National Police Agency. It attributed the decline largely to legislation passed to combat organised crime that includes measures like barring members of designated groups from opening bank accounts, renting apartments, buying cell phones or insurance.

Yakuza once operated from well-marked offices, often with signs out front and symbols of their trade such as lanterns and samurai swords visible through the windows. They were often portrayed in films and cartoons as noble outlaws with a code of honour. Their typical rackets were extortion, gambling, prostitution, gunrunning, drug trafficking and construction kickbacks.

But gang violence in a number of neighbourhoods, including the 2007 fatal shooting of Nagasaki Mayor Iccho Ito during his election campaign, have since led the government to tighten gun control, racketeering laws and other anti-gang measures.