By Sergio Spagnuolo

CURITIBA, Brazil (Reuters) - Brazilian police seized documents and questioned suspects on Thursday to investigate Panama's FPB Bank in connection with a sweeping graft probe of political corruption at state-run companies.

Police said FPB Bank was under investigation for "financial crimes, laundering of assets and transnational criminal organization" for offering private banking services without the authorization of Brazil's central bank.

Offshore companies registered by the Panama-based law firm Mossack Fonseca, the subject of a massive data leak this year, allegedly helped FPB Bank get clients' money out of Brazil illegally, police said in a news conference.

Funds came in part from a multibillion-dollar price-fixing and bribery scheme at Brazil's state-run oil company Petroleo Brasileiro SA, prosecutors added in a statement.

"Staff of the Panama-based bank in Brazil not only maintained a clandestine operation but also ensured their clients' anonymity by using offshores," federal prosecutors said.

An FPB Bank executive, Edson Paulo Fanton, was taken into custody for questioning in Santos, a port city near Sao Paulo, police said. Representatives for FPB in Brazil and in Panama did not respond to a request for comment.

Police said other banks are being investigated but declined to disclose which ones. The probe may widen depending on evidence to include other crimes, police officer Rodrigo Sanfurgo told the press conference.

Prosecutors said they have identified 44 offshore companies registered by Mossack at FPB's request. The law firm's representatives did not immediately respond to a request for comment.

A central bank spokeswoman confirmed FPB Bank is not authorized to operate in Brazil.

Thursday's operation also included search-and-seizure raids in Sao Paulo, Santos and Sao Bernardo, strongholds of suspended President Dilma Rousseff's Workers Party, which ran the country for 13 years starting in 2003.

The two-year-old Petrobras investigation, Brazil's biggest graft probe, has resulted in the jailing and conviction of dozens of powerful executives and politicians for fixing contracts in return for political contributions and personal bribes.

Fallout from the scandal has overturned Brazilian politics, feeding the movement to impeach Rousseff on unrelated charges of breaking budget laws. She denies any wrongdoing in that case or the Petrobras scandal.

(Reporting by Sergio Spagnuolo; Additional reporting by Silvio Cascione in Brasilia and Enrique Andres Pretel in Mexico City; Editing by W Simon and James Dalgleish)