Spokesman Jim Margolin would not say what the agents were looking for at Ramapo Town Hall in Suffern.
"We're executing a search warrant," he said. "It's under seal and not a matter of public record."
Town police and investigators from the Rockland County district attorney's office were also on the scene as agents wheeled empty, flattened boxes into the building.
District Attorney Thomas Zugibe would say only, "We're assisting the FBI in the execution of warrants at Town Hall."
Town workers were asked to leave as agents arrived at about 3:30 p.m., Margolin said. "The search will be easier to accomplish without people milling around," he said.
He said agents would likely be at Town Hall well into the evening.
Margolin said the warrant came from a federal court in New York but he would not say if the search was part of a new investigation or an existing case.
Mayor Noramie Jasmin and Deputy Mayor Joseph Desmaret of Spring Valley, a village within the Town of Ramapo, are among six defendants in an ongoing political corruption case at the federal courthouse in White Plains. That case centers on a plan to buy a line on New York City's mayoral ballot for state Sen. Malcolm Smith. All the defendants have pleaded not guilty.
Herb Hadad, a spokesman for the U.S. attorney's office, would not comment on the investigation.