Agents in dark blue FBI windbreakers carried white cardboard boxes out of the house, which is about a mile from Atlantic City's casinos. Earlier they removed a computer and a printer, stashing the items in dark sport utility vehicles.
Outside the home, FBI agent Jessica Weisman said, "We are here in an official capacity, executing a search warrant. That's all we can say."
IRS agents also were on the scene, a spokesman for the service's criminal investigations unit said Monday.
Gilliam was seen leaving his home on Monday. He refused to comment.
Gilliam, a Democrat, is in his first year as mayor. The former two-term city councilman defeated Republican incumbent Don Guardian in November 2017 in a contentious election marked by allegations of voter fraud by Guardian.
In April, a judge threw out a criminal complaint against Gilliam over the alleged theft of a $10,000 check from the Atlantic City Democratic Committee. The committee had accused Gilliam and a former campaign manager of unlawful taking when Gilliam deposited the committee's $10,000 check into his campaign account.
Gilliam said it was a mistake and later returned the money.
Last month, Gilliam and several other people were involved in an early morning fight outside an Atlantic City casino that was caught on surveillance video.
Local prosecutors announced last week they wouldn't file criminal charges, though citizen's complaints filed against Gilliam and Councilman Jeffree Fauntleroy were pending in municipal court.
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