VILNIUS, Lithuania (AP) — Lithuanian authorities are on high alert after a massive data leak involving more than 600,000 entries from national data registers, which is believed to have been executed by another country.

The Lithuanian general prosecutor’s office on Friday announced the leak was primarily from registers of real estate and legal entities accessed by using login credentials of institutions authorized to receive the data.

The head of the State Enterprise Centre of Registers, Adrijus Jusas, resigned Monday following the leak.

The authorities immediately implemented additional cybersecurity measures, including blocking the accounts of suspected data users and restricting access with a requirement to update credentials, the prosecutors said.

The prosecutor’s office said a foreign country is suspected of involvement, although authorities did not specified which nation.

Lithuanians are especially cautious given that the country, with a population of 2.9 million, is one of the main targets of Russia’s hybrid war against Europe, which includes sabotage, arson attacks and vandalism, as well as influence operations.

Opposition politician Laurynas Kasčiūnas wrote on social media Sunday that the data theft is suspected to be a Russian intelligence operation, although he offered no evidence for the claim.

The politician warned that addresses of intelligence officers, military personnel, diplomats or politicians may have been accessed, which could potentially allow the perpetrators to spy on or exercise pressure against the targets.