RIO DE JANEIRO—Swiss authorities on Wednesday said they are investigating units of the Brazilian construction firm Odebrecht SA in connection with a corruption scandal involving Brazil's state-run oil firm Petró leo Brasileiro SA, broadening an international money-laundering investigation.

The Swiss attorney general's office alleges, in a statement, that Odebrecht units paid bribes from accounts in Switzerland into separate Swiss accounts held by former Petrobras directors. Swiss authorities didn't name specific Odebrecht operating units or the Swiss banks involved.

Swiss authorities said they have requested help from Brazilian authorities to question suspects and "secure relevant documentary evidence" in Brazil.

The investigation in Switzerland adds to the pressure on Odebrecht after its chief executive, Marcelo Odebrecht, was arrested last month by Brazilian authorities on suspicion of money laundering and corruption in connection to the Petrobras case.

Mr. Odebrecht hasn't been charged with a crime. He is jailed in the southern Brazilian city of Curitiba under "preventative arrest," a tool Brazilian authorities have been using during the case. Through his company, he has denied wrongdoing.

Odebrecht, based in Salvador, Brazil, said it would contact the Swiss authorities to "understand the scope of the investigation and the reason why [Odebrecht] is being inserted in the context of irregularities committed by Petrobras executives."

"Odebrecht has every interest in clarifying the matter, but can only express its views after having access to information," the company said.

The company has previously denied any wrongdoing and said that the arrest of Mr. Odebrecht was unnecessary because the company was cooperating with authorities. Petrobras says it was a victim of the alleged scheme and that it is cooperating with authorities.

Last year, Brazilian investigators unearthed a yearslong corruption scheme in which they allege some of Brazil's largest construction firms formed a cartel to drive up prices of Petrobras contracts and kicked back some of the windfall to Brazilian politicians, top political parties and corrupt executives at the oil firm.

Several Petrobras executives stashed some of the money in Swiss accounts, according to Brazilian prosecutors. One former executive, Pedro Barusco, earlier this year returned nearly $100 million from Swiss accounts that he amassed through the bribery scheme. Mr. Barusco, who has cooperated with authorities and admitted to taking bribes in public testimony earlier this year, hasn't been arrested or charged with a crime.

After the revelations in Brazil, Swiss authorities opened their own investigation. In March, Swiss authorities froze roughly $400 million in assets tied to the scandal and agreed to send $120 million in frozen assets to Brazil. They had unearthed more than 300 accounts at more than 30 Swiss banks they said were used to hold ill-gotten funds.

"The initial results of these investigations indicate that the Swiss financial center has been seriously affected by the scandal, as numerous persons and companies that have been charged or indeed already convicted in Brazil conducted suspicious transactions involving accounts in Switzerland," the Swiss attorney general's office said Wednesday.

Swiss authorities said the Petrobras scandal "has led to a far more than average number of reports of suspicious transactions" from Swiss banks to a Swiss government office made available for reporting suspected money laundering.

Brazil's attorney general Rodrigo Janot traveled to Switzerland in March for a meeting with his Swiss counterpart and the two parties agreed to cooperate "in order to clear up this corruption scandal and bring the perpetrators to justice."

Four former Petrobras executives have been arrested, and roughly 50 current and former Brazilian politicians are under investigation for taking part in the scheme.

Some have admitted wrongdoing in exchange for lesser punishment, prosecutors say, while others have denied involvement and await trial.

This week, three former executives from another one of Brazil's largest construction conglomerates, Camargo Corrê a SA, were sentenced for money laundering and other charges for their roles in the scandal. Camargo Corrê a said it has offered to collaborate with authorities and is improving its corporate governance and control systems.

Write to Will Connors at william.connors@wsj.com

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