Greene was fired from PHA in September 2010 after almost 13 years as head of the nation's fourth-largest public housing agency. During his tenure, Greene used public money and raised private capital to replace antiquated public housing projects with new communities.
As a government-funded agency, PHA is prohibited from using federal money to pay lobbyists. Although lobbying isn't barred, public housing agencies must disclose any money used to lobby members of Congress.
HUD said PHA violated those rules by spending federal money on lobbying and failing to disclose it. The agency cited more than 30 incidents in which lobbyists working for PHA met with congressional staffers or members, including U.S. Sens. Robert P. Casey Jr. and Arlen Specter, and House representatives from Philadelphia.
In Thursday's letter, Clemmensen said Greene had been given an initial 30 days to respond to HUD's charges and then given an extension until March 20. He said Greene failed to meet the deadline.
Bergstrom, a former federal prosecutor now specializing in white-collar defense work, could not be reached for comment Friday.
Clifford Haines, Greene's civil lawyer, said HUD's allegations were without merit.
"At best, HUD has a document that was incorrectly filled out by someone at PHA. Carl Greene's signature on that document establishes no wrongdoing on Carl's part," he said.
As for Greene's missing the deadline, Haines said, "the debarment charges were responded to well before the due date."
HUD spokesman Jereon Brown said the federal agency had "done our due diligence, and I think we're doing what is best for the public."
PHA's main federal lobbying firm for years was the American Continental Group in Washington, records show. Its president has said he was regularly in contact with Greene about his firm's PHA lobbying work. The firm filed required disclosures stating it was lobbying for PHA, records show.
PHA's lobbying was first reported by The Inquirer in January 2012. Less than two weeks after the newspaper report, the FBI and the U.S. Attorney's Office sought PHA's lobbying files, records show.