Switzerland’s top prosecutor is pursuing at least one bank in the country for its role in the Brazilian corruption affair known as the Carwash. Michael Lauber’s biggest problem expanding the probe is that the banks hire pretty good lawyers.

Swiss law-enforcement officials have opened more than 100 criminal proceedings tied to the so-called Carwash scandal in South America after some Swiss banks were used to funnel bribes from construction firm Odebrecht SA to officials at Brazil’s state-run oil producer Petrobras. So far these include one bank in Switzerland, Lauber, the Swiss Attorney General, told reporters at his annual press conference in the national capital Bern.

“Right now, it’s about one bank in Switzerland, more could come,” Lauber said. “I can’t give you numbers now though. The banks have just about the best lawyers in Switzerland.”

Lauber’s office has seized more than 1 billion Swiss francs ($1.03 billion) in assets stashed in the country’s banks in the wake of investigations by Swiss, Brazilian and U.S. authorities, and has since returned more than 200 million Swiss francs to Brazil, according to an annual report released Friday. Odebrecht agreed to a $4.5 billion penalty at the end of 2016 to resolve bribery allegations involving Petroleo Brasileiro SA, as it’s formally known, though that number may yet come down given the company’s financial difficulties.

In 2015, the Swiss said they froze $400 million in assets from more than 30 banks in Switzerland for their suspected ties to the Petrobras scandal, without naming the banks. The following year, Lauber said he and his team of prosecutors were looking at account data from up to seven Swiss banks to see if any of them were criminally negligent in how they handled money related to the Petrobras affair.

He declined on Friday to name the bank that is now being prosecuted.