More than 10 people who visited Tokyo's Yoyogi Park or its vicinity might have contracted dengue fever, Fuji TV reported Monday.
Last week, three people commuting to the same school in Tokyo, including a teenage girl, and a man and a woman in their 20s, were found to have contracted the infectious disease in what are the first domestically transmitted cases in nearly 70 years.
They are suspected of having been bitten by a mosquito carrying the dengue virus.
About 200 people come down with dengue fever in Japan every year, but they are believed to have contracted the virus while traveling overseas.
The current 10-plus victims had never traveled abroad but all had visited Yoyogi Park recently, the broadcaster reported.
The authorities said they would try to confirm if the suspected cases are in fact dengue fever around noon on Monday, according to the report.