Scammers from Taiwan may have used vacant houses in Japan for telephone fraud targeting people in China.
Last month, Japanese police arrested 10 Taiwanese men and women at Narita airport near Tokyo for allegedly lying on their immigration forms.
Some of those arrested reportedly admitted to investigators that they had come to Japan to make phone calls as part of a scam.
Police believe the group bought or rented houses in Chiba and Yamanashi prefectures and used them as bases of operations.
Police searched the houses and found manuals written in Chinese, as well as cell phones and lists of names.
The manuals reportedly instruct callers to impersonate Chinese civil servants and tell victims that their personal information had been abused.
Police suspect that the intensifying crackdown on such fraud in China may have prompted scammers to come to Japan.
China has seen an uptick in fraud involving telephone calls and email.
Authorities confirmed about 596,000 such cases in 2017 and estimated the damage at more than 1.96 billion dollars.