This is the third such incident over the past 12 months.
Bulgaria's Chief Prosecutor Ivan Geshev has notified the foreign minister of evidence gathered regarding espionage efforts by "two foreign nationals enjoying diplomatic immunity."
That's according to a statement released on the agency's website.
After receiving the relevant signals from SANS security service, the Specialized Prosecutor's Office instituted pre-trial proceedings targeting "foreign nationals gathering information constituting a state secret for the purpose of handing it to a foreign state or organization."
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The information was then to be passed to Russia's military intelligence in Moscow, the report says.
The two suspects developed contacts with Bulgarian citizens who had access to information related to the defense industry. In certain cases, financial benefits have been "promised and provided" to Bulgarian citizens for such clandestine cooperation.
"It has been established that the two Russian citizens have diplomatic status," the report reads.
The suspects' diplomatic immunity does not allow Bulgarian authorities to prosecute them so the two spies were declared personae non-gratae and told to leave country within 72 hours, Reuters reported citing the foreign ministry.
Russian spies in Bulgaria: background