Police on Friday referred to prosecutors the case of a suspected leak of restricted information in 2013 by a former Ground Self-Defense Force commander to a Russian intelligence officer formerly posted in Tokyo as a military attache.

Papers on five former and incumbent Self-Defense Forces officers were also sent to prosecutors on suspicion they obtained an SDF booklet describing the operations of its divisions and brigades at the request of the former commander, Kazushige Izumi, 64, in violation of confidentiality provisions under the law governing the SDF.

The former commander of the GSDF's Eastern Army, a unit overseeing Tokyo and other prefectures in eastern Japan, is suspected to have offered the SDF textbook to 50-year-old Sergey Kovalev, believed to be a member of the country's military intelligence body GRU. He used to work as a diplomat at the Russian Embassy in Tokyo.

Investigative sources said the Russian intelligence officer has already returned to Russia. The Public Security Bureau of the Metropolitan Police Department has requested through diplomatic channels that Kovalev be summoned but Moscow has yet to respond, they said.

Izumi has admitted to asking his subordinates in May 2013 to acquire the booklet on the SDF's operations and battle tactics to give to the Russian agent.

Kovalev is suspected to have asked Izumi to get the booklet. No monetary transactions have been confirmed in the case.

Defense Minister Gen Nakatani expressed disappointment over the case, saying it could ruin public trust in the SDF, and pledged to fully cooperate with investigations.