Brazil's President Dilma Rousseff shows a document confirming former president Luiz Inacio Lula da Silva's appointment as chief of staff, at Planalto palace in Brasilia, Brazil, March 17, 2016. [Photo/Agencies]

"Those telephone conversations provide evidence of obstructing the investigation, part of a treacherous underground war waged in the shadows, far from the courts," wrote thirteen federal prosecutors on Moro's task force in a public statement.

Moro said in a court filing released on Wednesday that the taped telephone conversations did not provide proof that Lula and Rousseff were interfering with his investigation.

He said, however, that he released the recordings because citizens had a right to know how they were being governed.

One recording, made public by the court, included Rousseff offering to send Lula a copy of his appointment urgently "in case it was necessary" - a possible reference to the ministerial post providing him with immediate immunity from arrest.

Brazil's biggest-ever corruption probe, which centers on bribes and political kickbacks at Petrobras, has led to the conviction of dozens of powerful executives and politicians while recovering 2.9 billion reais ($795 million) in stolen money.