At least one Petrobras employee was believed to be on the drilling rig involved in the spill and prosecutors want to determine if Petrobras acted according to the law, Estado said.

Prosecutors are already investigating Chevron and Transocean, the owner of the Sedco 706 floating rig involved.

The spill occurred after unexpected pressure from an undersea oil reservoir in the Frade field northeast of Rio de Janeiro damaged the well more than 500 meters below the seabed and caused an estimated 2,400 barrels of oil to seep up through undersea rock and the seabed and into the ocean, Chevron said.

While Petrobras owns a 30 percent stake in the field, Chevron, with a 52 percent stake and the role of operator, has assumed responsibility for the accident and has taken the brunt of criticism for the spill.

The remaining 18 percent is owned by Frade Japao, a Japanese group. Chevron officials said November 20 that Transocean had no responsibility in the spill. The spill, tiny in comparison to the estimated four-million-barrel Deepwater Horizon spill in the Gulf of Mexico in 2010, has been totally dispersed without coming near Brazil's coast, according to Chevron. The Deepwater Horizon rig was also owned by Transocean.

Before Brazilian courts and environmental agencies, all owners of an oil field or exploration concession are co-responsible for any damages or accidents in their field according to Brazil's oil regulator, the ANP, and Marilda Rosado De Sa Ribeiro, an oil legislation expert at the Rio de Janeiro law firm of Doria, Jacobina, Rosado, Godinho.

At the ANP, though, all regulatory compliance issues are directed at the operator, ANP officials said. Chevron has already been fined $28 million for the spill and faces further fines, the ANP and Brazilian environmental officials said.

A spokeswoman for Petrobras chief executive Jose Sergio Gabrielli declined to comment on the probe or on the Estado report when called by Reuters on Saturday.

A well producing about 10 percent of the 79,000 barrels a day of oil pumped from the field was shut by the country's oil regulator, the ANP said December 1. The closure was the result of Chevron allegedly failing to report the presence of corrosive, sulfur-based gasses in the production stream.

(Writing by Jeb Blount; Editing by Nick Macfie)