Alleged members of a purported Russian spy ring were detained in Siauliai, Lithuania, after being suspected of infiltrating the Lithuanian army. The accused spies were captured Wednesday near NATO’s Zoknai air base, a facility that offers protection to Estonia, Latvia and Lithuania.

While authorities have described the espionage as the work of an unidentified “foreign intelligence service,” media speculation is that they were working for Russia, Newsweek Europe reported.

Lithuanian Lt. Col. Vidmantas Raklevicius told the Russian news agency Tass that one of the detainees was one of his own officers, who had been in the Lithuanian air force for 10 years. Another man, believed to be the ringleader, has also been detained.

Lithuanian authorities have launched an inquiry into the apparent breach, and officials have not ruled out Russia as its source.

The incident comes at a time when more Russian spies are believed to be operating in Eastern Europe than have been since the Cold War ended.

Moscow is thought to have ramped up its undercover espionage while it pursues apparently expansionist policies in Crimea and Ukraine -- with violent consequences.