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HELSINKI — Estonian officials said Wednesday that a former military officer and his father have been arrested on suspicion of treason and spying for Russia.

The state prosecutor's office and the Estonian Internal Security Service, or the security police, said in a joint statement that Deniss Metsavas, born in 1980, and Pjotr Volin, born in 1953, were detained Sept. 3.

The men are suspected of passing Estonian state secrets and other classified information to Russia's military intelligence service GRU for a period of more than five years.

Metsavas, a fluent Russian-speaker, served with Estonia's armed forces for years in various positions including a mission with NATO troops in Afghanistan. He advanced to the rank of major in 2015 and was lately posted at the armed forces headquarters in Tallinn, the capital.

Estonian news portal Delfi reported that his father Pjotr Volin served with the Soviet military between 1971-1979 but isn't otherwise a known figure in the country.

Prosecutor Inna Ombler declined to reveal what kind of information the two men - who according to local media received Estonian citizenship in the 1990s - passed on to Moscow but stressed they seemed to have worked seamlessly together.

"It is clear that it was Metsavas who had access to classified information in the first place but the forwarding of classified information and bringing it to the GRU was committed by both individuals," she told reporters.

The revelation of Estonia's latest spy case was seen as a serious blow for the small Baltic nation and NATO member of 1.3 million.

Commander of the Estonian armed forces Gen. Riho Terras called Metsavas a competent and committed officer "but he is a traitor" who caused significant damage to Estonia and its NATO allies which have been notified of the case.

"Damage has been done, a wound has been inflicted in the defense capability of Estonia," Terras said without elaborating.

Estonia, a former Soviet republic, has witnessed several high-profile spy cases since its independence in 1991 with both Estonian and Russian citizens being caught red-handed providing sensitive information to Moscow.

Russia has maintained high interest in the internal affairs of Estonia and its Baltic neighbors Latvia and Lithuania, particularly on military and security matters, after they joined NATO in 2004.