For the second time in three days, a former campaign staffer to District of Columbia Mayor Vincent Gray has pleaded guilty to a federal offense arising from Gray's 2010 mayoral bid.

Howard Brooks pleaded guilty Thursday to lying to the FBI about payments he made to another mayoral candidate using Gray campaign funds. On Tuesday, former Gray aide Thomas Gore pleaded guilty to making some of the same payments and shredding records of them.

Authorities said the cases makes clear that the Gray campaign engaged in dirty politics.

"Today's guilty plea further reveals the underhanded dealings that tainted the integrity of the 2010 mayoral campaign," U.S. Attorney Ronald Machen said in a statement.

What remains unclear is whether Gray participated in or even knew about the criminal activity. While Gray has suffered politically from the scandal, he has not been implicated in any crimes. He has insisted previously during a long-running federal probe that he knew nothing about the potential misdeeds committed by staffers.

The most serious offenses that arose from the cases against Gore and Brooks occurred after Gray took office and involved attempts to conceal the Gray campaign's schemes. Gore pleaded guilty to shredding records of payments made with Gray campaign funds to Sulaimon Brown, a minor mayoral candidate. And Brooks admitted lying to the FBI about his involvement in giving Brown the money.