Brazilian state-controlled oil giant Petrobras said it fired a senior executive who recommended the costly purchase of a Texas refinery in 2006, a deal that is being investigated for alleged irregularities.

Nestor Cuñat Cervero will be temporarily replaced as chief financial officer of Petrobras Distribuidora by the current president of that fuel-distribution subsidiary, Jose Lima de Andrade Neto, the company said in a statement.

The acquisition of the refinery has triggered controversy because it cost Petrobras hundreds of millions of dollars, far above the previous purchase price for the same production facility.

The scandal has even touched President Dilma Rousseff, who was predecessor Luiz Inacio Lula da Silva's chief of staff and a member of the oil giant's board of directors at the time of the purchase.

A group of five senators said Thursday they would formally ask federal prosecutors to probe Rousseff's role in that transaction, which is already being investigated by Congress and other federal bodies.

Rousseff said this week that Petrobras' board approved the deal based on a "legally flawed" report prepared by Cuñat Cervero, who was Petrobras' international director at the time.

Petrobras paid $360 million for a 50 percent stake in the Pasadena, Texas-based refinery in 2006, a year after Belgium's Astra Transcor Energy had acquired a full interest in the production facility for $42.5 million.

Due to contractual obligations, Petrobras spent an additional $820 million to acquire Astra's remaining stake several years later.