Peter Magyar, 43, a lawyer formerly close to the government who plans to launch a new party to challenge Prime Minister Viktor Orban, on Tuesday published a recording of a conversation with Varga, then his wife and Hungary's justice minister, in which she detailed an alleged attempt by aides to Orban's cabinet chief to alter documents in the case.
Chief prosecutor Pal Furcht told a briefing that prosecutors had retained and assessed Magyar's audio tape, presented as part of his witness statement, and that an investigation was under way into possible "failure to report bribery."
He said Varga had been heard on Wednesday.
Thousands of people protested near the parliament in Budapest on Tuesday, demanding Orban resign, after Magyar published the audio tape on his Facebook page.
Furcht said another witness was being heard on Thursday, but declined to name them or give any details about the statements made by Varga.
Asked if Orban's cabinet chief, Antal Rogan, would also be questioned, Furcht said he could not reveal any names as the investigation was under way.
The graft case centres on former justice ministry state secretary Pal Volner, who was charged in 2022 with accepting bribes from the former head of the Court Bailiffs, Gyorgy Schadl. Both have pleaded not guilty. Prosecutors are seeking jail terms for the pair.
The probe comes at a politically sensitive time for Orban ahead of European parliamentary elections in June, and follow a sex abuse scandal that brought down two of his key political allies - the former president and Varga - last month.
Varga could not be reached for comment earlier this week. She did not dispute the authenticity of the tape in a post on her Facebook page.
Government spokesman Zoltan Kovacs did not immediately respond to an emailed request for comment.
State prosecutors said in an earlier statement that it was "legally and physically impossible" to interfere with prosecution documents.
(Reporting by Krisztina Than; Editing by Kevin Liffey)
