An FBI spokesperson confirmed agents executed the warrant, but no other details were released.
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 emerged from the home shortly after 12:30, ignoring shouted questions from reporters. He then got into his personal sport utility vehicle and drove away.
The raid comes days after prosecutors said they will not pursue criminal charges against Gilliam and Atlantic City Councilman Jeffree Fauntleroy in regards to a brawl last month outside the Golden Nugget Casino.
It's not clear why the two went toe-to-toe with a group, which also involved women, but it was all caught on camera.
There is no word at this time on whether the raid is connected to this incident.
A man driving a car with the Atlantic City logo on its sides drove slowly past Gilliam's home, extending his middle finger to reporters gathered outside.
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.
(The Associated Press contributed to this report)
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