Italy says it expelled two Russian diplomats and arrested an Italian navy captain Tuesday for their alleged involvement in espionage.
The diplomats were expelled Wednesday, according to news reports.
Italian police say the captain and a Russian Embassy official were arrested in a parking lot in Rome and were accused of "serious crimes tied to spying and state security."
Reuters reported that an Italian police official told them the captain was named Walter Biot and that he was accused of passing information in exchange for $5,900.
Italian news agency Ansa reported that some of the documents seized were NATO documents.
Italian police said the arrests were the result of a lengthy investigation by national security and military officials.
After the arrests, Italy summoned the Russian ambassador, and two Russian officials allegedly involved in the incident were expelled.
Italian Foreign Minister Luigi Di Maio characterized the incident as "extremely grave," Reuters reported.
"During the convocation of the Russian ambassador to Italy at the Foreign Ministry, we let him know about the strong protest of the Italian government and notified the immediate expulsion of the two Russian officials involved in this extremely grave affair," the minister's Facebook post said, according to CNN.
Biot, 54, was reportedly working at the defense ministry in a department charged with developing national security policy and maintaining relations with Italy's allies, Reuters reported.
According to Reuters, Russian news agencies said the two expelled officials worked in the embassy's military attaché office. They did not say if the person arrested in the parking lot was one of those expelled.
Russian news agency Interfax reported that a Russian politician said it would reciprocate for the expulsions.
But a Kremlin spokesman downplayed the incident.
"At the moment, we do not have information about the reasons and circumstances of this detention," Kremlin spokesman Dmitry Peskov said, according to CNN. "But in any case, we hope that the very positive and constructive character of Russian-Italian relations will be preserved."
Both Bulgaria and the Netherlands have expelled Russian officials over spying allegations in recent months.