Federal prosecutors who uncovered a huge corruption scandal at oil company Petrobras are looking into whether Brazil's former president Luiz Inacio Lula da Silva received undue favors from engineering companies they are investigating.

In a letter to the Supreme Court made public on Monday, the head of the investigation, Deltan Dallagnol, argued for a federal probe because some of the alleged gifts were made while Lula was still in office.

The prosecutors suspect favors were extended to Lula by executives of engineering companies Odebrecht and OAS that have been charged with corruption and money laundering in the massive bribery and political kickback scandal involving contracts with state-run Petroleo Brasileiro SA.