(Bloomberg) -- Police in Slovakia, a NATO and European Union member bordering Ukraine, detained four people suspected of spying for Moscow and expelled three Russian diplomats in response.

Russia paid the suspects “tens of thousands of euros” for sensitive or classified information, Special Prosecutor Daniel Lipsic told journalists on Tuesday, a day after the detentions and the diplomats’ expulsion.

Two people were charged, including the former deputy head of the military academy in the northern city Liptovsky Mikulas, who confessed to “receiving financial reward for providing information” to Russia’s GRU military intelligence service since 2013, acting Police Corps President Stefan Hamran said.

“The investigation will reveal to what extent the interests of the Slovak Republic or the NATO alliance were endangered,” Hamran said.

The other person facing charges is a contributor to the website Hlavne Spravy, which Slovak authorities have labeled a vehicle for spreading disinformation. Authorities said the other two detained -- a former member of the Slovak intelligence agency and a lawmaker’s assistant -- had also helped secure sensitive information for Russia. They were released.

Russia’s Foreign Ministry said that the expulsion of its diplomats “would not go unanswered.” 

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