A third aide to D.C. Mayor Vincent Gray's 2010 campaign is expected to plead guilty Tuesday to felony charges amid an expanding probe that appears to be drawing closer to the mayor.

Jeanne Clarke Harris, whose communications firm consulted for the Gray campaign, was charged with federal counts of conspiracy and making false statements. Authorities also charged her with a local conspiracy count.

The three charges combine to carry up to 8 1/2 years in prison and $505,000 in fines, though Harris would likely receive a less severe punishment under federal sentencing guidelines.

The charges were filed in a criminal information, a type of court filing that indicates a guilty plea is imminent. Harris is scheduled to appear in U.S. District Court at 1 p.m. Tuesday.

Neither of Harris' listed attorneys, Frederick Cooke or Mark Tuohey, responded to requests for comment.

Prosecutors said in the filing that Harris and an unnamed "co-conspirator No. 1" orchestrated straw donations to federal and local candidates between January 2008 and March 2012.

Although authorities did not identify the co-conspirator, Harris has close ties to Jeffrey Thompson, a prolific contributor to political campaigns in the District and a major city contractor. Federal agents raided her home and office on the same March night that they searched locations with links to Thompson.

The description of the co-conspirator -- "the sole owner of [a] company ... and the majority owner of [a] company" -- matched the background of Thompson, who owned a managed health care company and was the top partner in an accounting firm. Brendan Sullivan, Thompson's lawyer, was traveling Monday and unavailable for comment.

"Harris did knowingly and willfully conspire, combine, confederate and agree with others, including co-conspirator No. 1, to make contributions and cause contributions to be made in the names of others to various candidates for District of Columbia office," prosecutors wrote.

Prosecutors said that Harris and her co-conspirator worked out an agreement in which Harris would organize groups of contributors -- including family members, friends and employees -- to give to political campaigns. The co-conspirator later reimbursed Harris and, through her, the other donors.

The resulting tens of thousands of dollars in donations, authorities said, included $38,000 in contributions to a 2010 candidate for D.C. mayor. That candidate was Gray.

After Harris learned that the FBI was investigating the Gray campaign, she and her co-conspirator "arranged to take steps to impede federal officials from obtaining information concerning their involvement."

"Prosecutors also said Harris deducted $908,217 in political expenditures on the tax returns of one of her companies. Under federal law, political contributions are not deductible.

Legal trouble for Gray campaign aides has become commonplace in recent months. Two other staff members -- consultant Howard Brooks and assistant treasurer Thomas Gore -- pleaded guilty in May to covering up a scheme to pay Sulaimon Brown, a minor mayoral candidate, to remain in the 2010 contest so he could criticize Mayor Adrian Fenty, Gray's political rival.

But the charges against Harris demonstrate that authorities have moved beyond Brown's allegations. One person familiar with the case, who demanded anonymity to discuss the ongoing investigation, said that the probe of illegal, high-dollar donations is "where the big game is."

Whether that means Gray is in legal jeopardy is a question that has hung over the District's government for months, and queries about what the mayor knew about the illegal activity surrounding his campaign are likely to seize the spotlight once more. Last month, speculation about Gray's legal future became so rampant that he had to publicly deny he had plans to resign.

Robert Bennett, the mayor's criminal defense attorney, did not respond to requests for comment Monday.