The National Bureau of Investigation (NBI) was investigating information that NLB unlawfully gained EUR 257m after its liabilities related to subordinated bonds were extinguished due to decisions taken by "official persons" as part of the late-2013 bank bailout.

The sum corresponds to the amount of subordinated bonds of NLB that were wiped out as part of the bailout.

Media reports suggested that the suspects include central bank Governor Boštjan Jazbec, Vice-Governor Mejra Festić, former Vice-Governor Stanislava Zadravec Caprirolo and Tomaž Čemažar, the deputy head of the central bank's banking supervision department.

However, Festić rejected the claims and it later turned out that it was former Vice-Governor Janez Fabijan being investigated.

According to a report by news portal 24ur.com, the investigation ran late into the night with most of the suspects protesting over what investigators can take, as they have sensitive data on servers, e-mails, etc.

Draghi was informed of the investigation by Jazbec, which prompted the EU official into protesting with Slovenian State Prosecutor General Zvonko Fišer over "the illegal seizure of confidential ECB data".

Draghi also sent a letter to Juncker and urged him to intervene to allow central bank Banka Slovenije and its governor to carry out their tasks.

The European Commission confirmed Juncker had received the letter, stressing it was waiting "the results of the ongoing investigation in Slovenia and closely follows any developments". "It is too early at this stage for us to say more on this case."

The ECB president moreover said that given that EU legislation gave the ECB special immunity, Slovenian legislation would be checked to see if there were any relevant legal means to address the issue.

The Slovenian General Police Administration responded by saying that neither Banka Slovenije nor its employees were privileged when it came to preliminary criminal investigations.

"As regards the investigation, the suspects were not representing the EU. They were treated as employees or officials of a Slovenian institution," the police said.

What is more, in the past, Banka Slovenije never asked the ECB for permission to give over documents as part of preliminary criminal investigations, the police added in the press release.