Berman has not been sentenced in the case. Depending upon the level of cooperation he provides, he could be spared jail time. He agreed to a $750,000 fine.
The investigation into collusion and restraint of trade within the auctions has been going on for more than two years, with Nusbaum and Stollof identified as targets in a 2007 FBI affidavit.
FBI spokesman Rich Wolf said Tuesday that his agency played a large role in the investigation along with the Justice Department's antitrust division in Washington.
A Justice Department spokeswoman declined to comment.
The FBI affidavit alleges that Nusbaum, Stollof and Berman cooperated with one another to divvy up large blocks of available liens at tax sales throughout Maryland, using signals to let the others know when it was safe to bid.
An FBI surveillance team saw the men meeting at Berman's office in Baltimore County shortly before dropping off bid envelopes in Montgomery County, the affidavit says.
FBI records showed hundreds of calls between Berman and Stollof, who once worked for Berman's father at the old Fairfax Savings and Loan in Baltimore, and bid rigging between Stollof and Nusbaum in 1992, the affidavit alleges.
Thousands of properties with liens against them because of unpaid taxes or municipal bills are sold each year at annual auctions to investors, who often foreclose on the properties. Hundreds of homeowners have lost their houses because of minor debts such as water bills and rental property registration fees.
The Baltimore Sun reported in 2007 that most tax lien sales in the state were dominated by a few investors.
