SAO PAULO (Reuters) - Brazil’s top prosecutor on Monday asked the Supreme Court for permission to investigate an ex-president, two senators and the former head of a unit of the state oil company for alleged efforts to thwart the country’s largest corruption probe.

Brazil's Senator Jose Sarney speaks with Reuters during an interview in Brasilia January 23, 2013. REUTERS/Ueslei Marcelino

Prosecutor General Rodrigo Janot accused ex-President Jose Sarney, who now sits in the Senate, along with senators Romero Juca and Renan Calheiros of carrying out “political maneuvers” in a bid to quash the inquiry into political kickbacks at state-run oil firm Petrobras.

All are allies of President Michel Temer.

Janot also requested that Sergio Machado, the former head of Transpetro, the logistics arm of Petrobras, be investigated for his role in the efforts to hamper the investigation.

Under Brazilian law, sitting federal congressmen, some members of the executive branch and other officials can only be investigated with approval from the Supreme Court. The top court must then approve charges and try them.

Jucca and Calheiros, who was until last week the president of the Senate, are already under investigation by Janot for their alleged involvement in the Petrobras scheme.

Machado, who has turned state’s witness, told federal prosecutors that he taped hours of conversations with the others named on Monday in which they discuss how they could create roadblocks to the unprecedented investigation into political graft.

Machado told prosecutors that the group was considering, among other maneuvers, passing a law that would make it illegal for an imprisoned person to strike a plea bargain with prosecutors.

Several of the most important state’s witnesses arranged such deals after being jailed on corruption charges or convicted.

It is expected that over 100 sitting politicians could be investigated and possibly tried in connection with the Petrobras case and subsequent investigations into graft at state-run enterprises.

Major Brazilian construction firms have admitted to paying billions of dollars in bribes in return for lucrative government contracts.