An illegal shadow campaign helped elect Vincent Gray as D.C. mayor in 2010, federal prosecutors said Tuesday, further staining a campaign that has been rocked by the convictions of three operatives since May.

"The 2010 mayoral election was corrupted," U.S. Attorney Ronald Machen Jr. said. "The mayoral campaign was compromised by backroom deals, secret payments and a flood of unreported cash."

In court filings, prosecutors said the shadow campaign "was coordinated with members" of Gray's apparatus. Gray was not named in the documents, though prosecutors identified him as the candidate whose campaign benefited.

Prosecutors discussed the shadow campaign as Jeanne Clarke Harris, a onetime communications consultant for Gray, pleaded guilty to three federal and local charges.

Harris said an unnamed co-conspirator used her company, Belle International, as an intermediary to fund the shadow campaign, though another person designed it.

"The money was from co-conspirator No. 1, but the plan was developed by another person," Harris said in court.

Though Machen declined to name the co-conspirator, Harris has close ties to Jeffrey Thompson, a prolific contributor to the District's political campaigns. Federal agents raided Harris' home on the same night they searched locations linked to Thompson.

A description of the co-conspirator's business interests in court records matched those of Thompson, whose attorney has not responded to requests for comment.

The shadow campaign, prosecutors said, began after Harris learned from "individuals associated with Candidate A's campaign that the campaign needed financial assistance."

"Candidate A" was Gray.

But Harris said her co-conspirator feared retribution from Adrian Fenty, the incumbent mayor and Gray's archrival, if information surfaced that he was financing an operation to help unseat him.

The subsequent campaign -- designed to identify voters who were likely to support Gray, encourage them to vote and help them travel to the polls -- remained a closely guarded secret.

But investigators said the shadow campaign left a money trail that revealed its cost: at least $653,800.

Prosecutors said the money was used for payroll expenses and supplies, including tens of thousands of T-shirts, posters, yard signs and stickers "bearing logos, graphics and designs identical to those used by [Gray's] campaign."

Most of the items, Harris acknowledged, were delivered to Gray's campaign offices, but Machen refused to say if Gray is believed to have known about the arrangement.

After federal investigators launched a probe of Gray's campaign, Harris said she and her co-conspirator scrambled to mislead authorities by forging documents and amending Belle's tax returns.

Harris also said the co-conspirator believed to be Thompson tried to have her flee the United States at his expense for five years "in order to evade federal investigators." Harris turned down the request, but she agreed to take a three-month trip to Brazil. The visit was ultimately canceled.

The shadow campaign was a key component of the fraud and conspiracy charges that Harris pleaded guilty to as she admitted to a starring role in a multiyear plot to funnel money to the campaigns of Gray and other political candidates. In 2010, Harris helped the Gray campaign secure $38,000 in contributions.

But those contributions were not from Harris, her two companies or the 16 family members, friends and employees whose names were attached to the donations. Instead, the co-conspirator reimbursed Harris and, through her, the others for the donations.

Under the terms of her plea agreement, Harris, 75, faces up to 37 months in prison and $60,000 in fines. A status hearing is scheduled for October.

The investigation, however, is not finished. Machen said he is "committed to pressing forward with this investigation until we hold everyone who played a role in deceiving the voters in 2010 accountable for their actions."