Documents obtained by The Times of Trenton show agents also are investigating vendor contracts with the city.
Agents last week searched City Hall offices and the homes of Mayor Tony Mack, his brother and a campaign donor.
Mack's two years in office have been marked by accusations of cronyism and mismanagement. The Democrat's only public comment has been to say he has not violated the public trust.
Mack is out of town on vacation and is due to return on Monday.