Bulgarian officials said Tuesday that unknown hackers have stolen the personal data of millions of people from the country's tax agency and noted a possible Russian link in the case.

Prime Minister Boyko Borissov called an emergency meeting of all law enforcement agencies to discuss the potential harm to Bulgaria’s national security. Finance Minister Vladislav Goranov told reporters following the meeting that the hackers behind the breach contacted local media using a mailbox from a Russian domain.

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The leak contained names, personal data and financial earnings of individuals and companies. According to local media, hackers stole the details of some 5 million members of Bulgaria's 7 million people. Goranov added Bulgaria has since requested help from the European Union’s cybersecurity agency.

Bulgaria's Prime Minister Boyko Borissov arrives for a meeting of EPP members ahead of an EU summit. (The Associated Press)

Interior Minister Mladen Marinov told television station bTV that the cyber attack coincided with Bulgaria's purchase of U.S. F-16 fighter jets for its air force and could have been motivated by it.

“Organized criminal groups involved in cyberattacks usually seek financial profits, but here political motives are possible,” said Marinov. “The government decided yesterday to buy F-16 jets.”

Goranov, however, refuted a possible link to the jet purchase, saying the cyberattack had occurred before the deal was approved.

Bulgarian media, which received an e-mail from the hackers, said it came from Russian mail provider Yandex but demanded no ransom. The e-mail instead called for the release of WikiLeaks founder Julian Assange, who is now in a British prison serving a 50-week sentence for evading court. Assange also faces an extradition request by the United States, which seeks him on account of espionage charges.

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The hackers’ e-mail criticized the Bulgarian government, saying “the state of your cybersecurity is a joke.”

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It is not clear why the national revenue agency was targeted but corruption in Bulgaria is widespread.

Transparency International, an international non-governmental organization committed to combating global corruption, has stated Bulgaria is the most corrupt of the European Union’s 28 member nations.

Fox News' Morgan Cheung and the Associated Press contributed to this report.