The name "Jeffrey Thompson" has never publicly passed through the lips of U.S. Attorney Ronald Machen Jr.

But in a court filing and during a rare news conference this week, Machen left little doubt that Thompson, a longtime D.C. government contractor and political donor, and others suspected of campaign corruption remain squarely in his prosecutorial sights.

"If you doubt our resolve, don't," Machen said in a thinly veiled warning to local elected officials and their allies. "If you believe you're too smart or your scheme is too complex, you're wrong."

Thompson has been identified in court documents and public statements only as "co-conspirator 1" and has not been charged with any wrongdoing. But his role in the shadow campaign that helped elect D.C. Mayor Vincent Gray in 2010 is believed to be a critical focus of the ongoing criminal probe.

On Monday, prosecutors revealed that Thompson and his close associate Jeanne Clarke Harris funneled $20,000 in illegal contributions to Michael Brown's ill-fated 2007 campaign for a council seat.

"Co-conspirator 1 agreed to contribute to Brown's political campaign committee for the Ward 4 seat, but not in a public manner," prosecutors wrote.

Harris pleaded guilty in July 2012 to violating the District's campaign finance laws, though court records do not mention any misconduct linked to Brown's 2007 campaign.

Brown, who was not charged in connection with the donation scheme but pleaded guilty to a bribery count tied to a later stint as a legislator, is assisting investigators with their probe.

"He has been cooperating with the authorities, and I expect he will continue to do so," said Brian Heberlig, Brown's attorney.

Brown's cooperation is certain to increase the scrutiny of Thompson, whose home and offices were the targets of law enforcement raids in March 2012.

"Obviously, the investigation against Mr. Thompson has been underway for some time, and a lot of time and effort has gone into securing Mr. Brown's guilty plea," said Glen Donath, a former federal prosecutor. "I think one can reasonably conclude that if there's any way to link the Brown and Thompson investigations, the U.S. attorney's office would do so."

Although the authorities have successfully prosecuted three now-former city lawmakers for transgressions while in office, any charges against Thompson could have sweeping effects on the District's political scene.

Through the years, his campaign donations touched dozens of D.C. politicians and campaigns, including most sitting members of the D.C. Council.SClBThompson's legal team has mounted an aggressive defense, clashing with prosecutors repeatedly in closed court proceedings and through sealed filings about whether the authorities can review millions of pages of seized documents. Federal judges have repeatedly ruled in prosecutors' favor.

Thompson's lawyer, Brendan Sullivan, has refused to comment on the pending investigation.