The Petrobras logo is seen in front of the company's headquarters in Sao Paulo, April 23, 2015. Reuters/Paulo Whitaker

SAO PAULO -- Brazilian investigators believe 6.19 billion reais ($2.1 billion) in bribes were moved in a corruption scheme involving state-run oil firm Petroleo Brasileiro SA (PETR4.SA), prosecutor Deltan Dallagnol said on Thursday.

Prosecutors seek to restore 6.77 billion reais to public coffers through fines and the return of stolen funds, he said at a televised press conference to present criminal charges against 13 people, including four former congressmen.

Three of the ex-lawmakers were preventively detained on April 10, the first politicians arrested in the 14-month-old probe. One is from the ruling Workers' Party and the other three belong to the smaller opposition Party of Solidarity.

Dallagnol, speaking from the southern city of Curitiba where Brazil's largest-ever corruption investigation began, called the charges "emblematic" and said they showed the prosecution had entered the political nucleus of the kickback scheme.

Prosecutors accused former executives at Petrobras, as the oil major is known, and two dozen engineering firms of inflating the value of service contracts and funneling the excess funds into their own bank accounts and to political parties.

The Supreme Court in Brasilia is also investigating 34 sitting politicians on suspicion of receiving bribes but none have yet been formally charged. Elected officials enjoy special legal protection in Brazil.

One of the former congressmen charged on Thursday is suspected of taking bribes in exchange for helping a public relations firm and a biotech lab secure contracts with state-run bank Caixa Economica Federal and the health ministry.

Another is thought to be the main distributor of kickbacks to the Party of Solidarity.

All are tied to the Petrobras probe by Alberto Youssef, the black-market money changer who agreed to reveal beneficiaries of the corruption scheme in exchange for a lighter sentence.

Federal Judge Sergio Moro will decide whether or not to accept the charges. If he does, the former congressmen will join the 97 other people who have been indicted.