Brazil could cancel the acquisition or seek compensation, and Astra Oil, a division of Belgium-based holding Transcor Astra Group, could face consequences in Brazil and in the U.S., they said.

"The company that sold it paid bribes," prosecutor Carlos Lima said at a press conference in Curitiba. "This is a defect in the deal and I believe we can seek reparation."

Petrobras, a state-run company, is seeking to recover losses from all illicit activities and will continue collaborating with authorities, it said in an e-mailed statement. The company has carried out internal audits to detect graft and sent the results to the appropriate agencies, it said. Efforts to reach Astra's offices in the U.S. and Belgium by phone were unsuccessful.

The acquisition first came under criticism in 2012, when Petrobras obtained full ownership and evidence of overpricing came to light. President Dilma Rousseff was the chairwoman of Petrobras at the time of the initial transaction, and the deal became a subject of the 2014 presidential campaign. Brazil's audit court, known as TCU, ruled last year that the purchase cost Petrobras $792 million in losses. The public prosecutors' office cited the losses in a press statement Monday.

Petrobras agreed to buy 50 percent of the 100,000-barrel-a-day Pasadena refinery from Astra in late 2005, and bought the remaining stake in 2012, for a total of $1.2 billion. The trading firm only paid $42.5 million for the plant in early 2005 before selling it to Petrobras, according to company filings.

The nearly 100-year-old plant was known as "the redhead" inside Petrobras's international division because of its outdated equipment, Lima said.