At least 28 political candidates, including D.C. Mayor Vincent Gray and a major 2008 presidential candidate, benefitted from secret financing schemes coordinated by D.C. businessman Jeffrey E. Thompson, according to a guilty plea entered by Thompson in federal court.

Thompson pleaded guilty to two conspiracy charges on Monday, admitting to illegally channeling more than $3.3 million to local and federal-level candidates between 2006 and 2012.

“Election after election, Jeff Thompson huddled behind closed doors with corrupt candidates, political operatives, and businessmen, devising schemes to funnel millions of dollars of corporate money into local and federal elections,” said U.S. Attorney Ronald C. Machen Jr., who has been leading the investigation in coordination with the FBI and the IRS.

Gray was named by Assistant U.S. Attorney Michael Atkinson during the four-hour hearing in U.S. District Court, though other beneficiaries of Thompson’s schemes were referred to as “presidential candidate A” or “D.C. Council Candidate D.”

Gray denied the allegations and continues to insist he is not guilty of any wrongdoing. He has not been charged with any crimes.

“FBI will be unwavering in combatting corruption in the District of Columbia,” warned Valerie Parlave, assistant director in charge of the FBI’s Washington Field Office.

Machen urged those who have participated in “back room, under the table deals with Jeff Thompson” to come forward, and said in a news conference that the charges heard in court were only the “tip of the iceberg” in what has been discovered in the ongoing probe.

Pressed on whether Gray would be taken into custody, Machen emphasized that all cases must be solid, declined to talk about potential charges and said, “We’re still running down some loose ends.”

On the federal level, Thompson pleaded guilty to backing his choice for president in 2008 with more than $800,000 in corporate contributions that were never reported to the IRS. He funneled money to help his candidate in 2008 primary elections in Texas, Pennsylvania, Indiana, North Carolina and Puerto Rico. Troy White, a member of then-Sen. Hillary Rodham Clinton’s 2008 presidential campaign team, was previously linked to Thompson in the case.

The presidential candidate is not alleged to have had any knowledge of Thompson’s scheming with campaign organizers. He helped fund the canvassing efforts of street teams to disseminate posters, stickers and yard signs.

Under terms discussed in court Monday, one of the two charges will be dropped if Thompson complies with investigators. He would face no more than six months in prison, followed by three years of supervision. The plea agreement was approved by Judge Colleen Kollar-Kotelly.

With three weeks to go before the April 1 Democratic mayoral primary, Machen was asked if he had a message for D.C. voters.

“That’s not my job,” he said.

While Machen shied away from mayoral politics, Gray’s opponents were quick to pounce Monday with some predicting he would soon be brought to justice.