Two FBI agents delivered a subpoena seeking additional information at state Sen. Malcolm Smith's Queens office on Wednesday afternoon.
The move was part of the bribery extortion case that federal authorities are conducting on Smith, who was indicted for allegedly bribing Republican county chairmen so he could run as a Democrat on the GOP ticket for mayor of New York City.
Five other New York politicians were also indicted.
Key to the investigation is a cooperating witness who set up meetings between Smith and an undercover FBI agent.
DNAinfo.com identified the cooperating witness as Moses Mark Stern of Monsey, N.Y. In a separate case, Stern owes Citigroup $126 million, the company says.
The indicted mayor of Spring Valley, Noramie Jasmin, has asked for time before being judged.
Rep. Charles Rangel, who was investigated and censored for ethics violations, said Smith's arrest is a blow.
During the press conference announcing the indictments, U.S. Attorney Preet Bharara said New York politics has pervasive corruption.
With what appears to a continuing FBI investigation, a number of state legislators have said Smith should be removed from his committee appointments in Albany. Some have said he should step down.