SÃO PAULO—The alleged corruption scheme at Brazil’s state-controlled oil company,Petroleo Brasileiro SA, was limited to three top executives and not spread throughout the company, according to a witness testifying Thursday before a committee in the country’s Congress.

Shares of the company plunged at the opening bell Thursday after the company wrote off 50.8 billion Brazilian reais ($16.8 billion) in 2014 due to losses from alleged graft and overvalued assets.

The witness, Augusto Mendonça Neto, is president of Setal Engenharia, an engineering company that is a supplier to Petrobras, as the oil company is known. Brazil’s federal prosecutors and police are alleging that Petrobras executives conspired with some of its suppliers to inflate the cost of contracts, and the money was then distributed among the oil company executives and Brazilian politicians. The congressional committee is investigating the allegations. Many executives and politicians related to the investigation have denied wrongdoing.

Mr. Mendonça Neto has acknowledged being part of the alleged scheme, and is cooperating with investigators. He named three former Petrobras executives, Paulo Roberto Costa,Renato Duque and Pedro Barusco, as also being involved, and said the rest of the company isn’t corrupt.

“The only contact I had with corruption was with those three people I named, apart from that there was never anything,” Mr. Mendonça Neto told the committee. Petrobras “is a highly competent company, composed of people who are extremely prepared and competent.”

Mr. Costa and Mr. Barusco have admitted to being involved and are cooperating with investigators. Mr. Costa was sentenced to house arrest and to wear an ankle bracelet on charges related to the investigation earlier this week. His house arrest sentence will be finished in October 2016. Mr. Barusco hasn’t been arrested nor formally charged. Mr. Duque has denied any involvement, but was arrested in connection with the investigation in March and remains in custody.

The alleged corruption under investigation started in 1997, and continues until today, Mr. Mendonça Neto said, without elaborating on the current state of the alleged scheme.

Write to Rogerio Jelmayer at rogerio.jelmayer@wsj.com and Jeffrey T. Lewis atjeffrey.lewis@wsj.com