Senator Delcidio do Amaral, a former Senate leader for the ruling Workers' Party, yesterday said in an interview that embattled ex-president Luiz Inacio Lula da Silva masterminded the graft scheme at state oil company Petrobras and that Rousseff used some of the proceeds to fund her presidential campaigns.
"Lula directly negotiated the appointment of Petrobras's directors with the different parties in Congress and knew exactly what the parties did with the directorships, mainly in terms of financing their campaigns," Amaral told weekly magazine Veja.
An irate Rousseff ordered her government to press criminal charges against Amaral for his "defamatory statements," which she categorically denied.
It is the latest damaging allegation from the senator, who on Tuesday accused Rousseff of trying to buy his silence when he was detained in the Petrobras case.
He made that accusation as part of a plea bargain in return for a lighter sentence -- the kind of deal investigators have used repeatedly to implicate a steadily growing list of rich and powerful figures.
Brazilian politics has been upended by the scandal.
The crisis has triggered angry protests laying bare sharp divisions in Brazil.
Mass rallies for and against the leftist president have rocked the country in recent days, just months before it hosts the Olympics in Rio de Janeiro in August, with riot police firing stun grenades and water cannons to disperse anti-government demonstrators.
Rousseff's enemies are demanding her departure, while her supporters accuse them of attempting a "coup."
Government supporters mobilised on Friday as pressure mounted on Rousseff, 68, and Lula, 70.