Ukraine's National Anti-Corruption Bureau (NABU) has questioned Interior Minister of Ukraine Arsen Avakov about the so-called "secret ledgers" of the Party of Regions.

"As promised, we asked Avakov to visit NABU. The reason was pages from the so-called "secret ledgers" of the Party of Regions, which he handed over at the end of September," NABU's press service said on its Facebook page on Friday.

During the interrogation Avakov, following the legally established procedure, explained to NABU detectives in what manner and from where he had obtained the pages from the ledger.

"Unfortunately, we are not able to divulge the details, since they are a confidential part of the investigation," NABU's press service said.

As earlier reported, on September 29, 2016, Avakov turned over several original pages of the original ledger.

On September 30, head of the Specialized Anti-Corruption Prosecutor's Office Nazar Kholodnytsky said that NABU intended to question Avakov as a witness in the investigation of the "secret ledgers" case.

At the end of May 2016 information was released by the former first deputy head of Ukraine's Security Services (SBU), Viktor Trepak, corroborating illegal cash payments made by the Party of Regions to a number of former and current state officials. In his words, the "secret ledgers" detained payments of around $2 billion.

NABU initiated an investigation based on Trepak's statements pursuant to Party 4 of Article 369 of the Criminal Code (bribery).

Information in the documents appears to show that Party of Regions was engaged in the financing of other political forces, including those represented in the Verkhovna Rada of the eighth convocation, Prosecutor General Yuriy Lutsenko said then.