RIO DE JANEIRO (Reuters) - A top Brazilian prosecutor said Thursday there was still much work to be done to rid state-run oil company Petrobras of corruption, despite more than three years of investigations into a political kickback scheme involving contractors.

Prosecutor Carlos Lima made the statement at a news conference detailing the arrests of three former Petrobras executives who allegedly took 100 million reais ($32 million) in bribes. Some of the money held offshore was then brought into Brazil through a government amnesty program for foreign assets.

“Nothing guarantees that today we have a company that is truly cleaned of the corruption of its past,” Lima said in reference to Petrobras, also known as Petroleo Brasileiro SA.

Petrobras Chief Executive Pedro Parente, who took over a year ago, has said the firm is working hard to shake out corruption and that it must quickly remove any doubts in order to help the revival of the company, the most indebted in the industry.

Lima and other investigators also sharply criticized the amnesty program as an easy way for those involved in the Petrobras scheme to launder bribes held in overseas bank accounts.

Their comments ratcheted up tension between anti-corruption crusaders in Brazil’s judiciary and the executive and legislative branches, who have held the program up as a success.

Investigators alleged that of the executives arrested, none of whom were named, one had brought some 48 million reais of bribe money that had been in offshore accounts back into Brazil under false pretenses, taking advantage of the amnesty program last year.

“This is a very serious fact, as it shows that the law ... institutionalized the money laundering of assets held abroad,” said prosecutor Diogo Castor in a statement.

The criticism of a law passed last year with the support of President Michel Temer underscored how prosecutors have increasingly come into open confrontation with the political establishment.

Temer hailed the amnesty program for raising around 50 billion reais in taxes and fines to help the government meet its budget target and called on Congress to start another program swiftly. In March, the Senate sent a bill to his desk opening the repatriation window for another 120 days.

Last month, the Petrobras probe sent shockwaves through the capital when the Supreme Court authorized investigations into eight cabinet ministers and dozens of senior lawmakers.

Prosecutors reiterated on Thursday that they had presented Congress with 10 measures that should be passed into law to battle corruption.