A Japanese man in his 50s who was detained in China in March 2017 for alleged spying was indicted this month, informed sources said Saturday.

The man was detained in Hainan Province, where he was conducting research on hot springs in China.

Around the same time, five other Japanese were held in Hainan or Shandong province. Four of the six were eventually released, but the other two, including the man in question, were officially arrested in September last year.

Details of the charges are unknown. Chinese media have reported that the two, who were said to be in possession of maps and other confidential materials, are suspected of violating the anti-espionage law that took effect in 2014.

Many Japanese have been detained by China since 2015.