D.C. Mayor Vincent Gray files paperwork to run for reelection at the D.C. Board of Elections in 2013. (Cliff Owen/AP)

D.C. Mayor Vincent C. Gray rejected a proposal by federal prosecutors in September that he plead guilty to a single felony count in connection with their long-running investigation of his 2010 campaign, a person with knowledge of the talks said.

Prosecutors have also re-interviewed key witnesses in recent weeks, several individuals familiar with the case said — a further indication that authorities are marching toward an indictment of the outgoing mayor.

A deputy to U.S. Attorney Ronald C. Machen Jr. offered the plea deal in a mid-September meeting with Gray’s attorney, Robert S. Bennett. The person with knowledge of the talks said Gray (D) was not present for the plea offer, which would have required him to admit to one felony count. Bennett told the prosecutor that Gray would not accept any deal requiring him to plead guilty to anything.

In an interview Friday, Gray referred questions about the investigation to Bennett, but he steadfastly denied any guilt — as he has repeatedly over the 3 1/2 years investigators have spent examining his 2010 campaign.

“What would I plead to?” Gray said. “I’ve made clear all along what my feelings are about this whole situation. . . . So I’m not about to change anything. What would I plead to? I don’t know how to put it any other way than that.”

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Jeffrey Thompson

Bennett declined to comment except to say, “If indicted, he will vigorously defend against the charges and will seek a speedy trial.” William Miller, a spokesman for the U.S. Attorney’s Office for the District of Columbia, also declined to comment. “The investigation is ongoing,” he said.

Six people have pleaded guilty to federal crimes related to Gray’s mayoral campaign since 2012 — most recently Mark H. Long, the mayor’s campaign driver, who admitted in September to conspiring to violate campaign finance laws.

The most damning claims aired in court files and hearings focus on Gray’s alleged knowledge of a $653,000 “shadow campaign” on his behalf funded by businessman Jeffrey E. Thompson and executed by close associates Jeanne Clarke Harris and Vernon E. Hawkins.

The campaign was also dogged by revelations that a rival candidate, Sulaimon Brown, had been paid secretly by Gray associates to boost Gray’s chances by staying in the race and verbally attacking incumbent Adrian M. Fenty (D).

After Thompson pleaded guilty to two felony charges in March, Machen said, “The veil over political corruption in the District of Columbia has been lifted.” He said city residents “deserve the truth.”

He also said the investigation was not over — an indication interpreted widely as a sign that the mayor was headed for indictment.

Reporter Nikita Stewart explains Jeffrey Thompson's significance in D.C. politics and connections to key figures. (The Washington Post)

“I promise you, we are not going away,” Machen said at the time. “We are firm in our resolve.”

Gray has not been named in court documents or by Machen, but prosecutors have made references to his campaign in court hearings. The investigation played a crucial role in Gray’s political demise. Two weeks after Thompson’s guilty plea, Gray lost his bid for a second term in the Democratic primary to D.C. Council member Muriel E. Bowser (D-Ward 4). She went on to win the general election this month.

Prosecutors tendered the possible deal for Gray in a meeting just weeks after Long pleaded guilty in D.C. Superior Court. Since then, according to multiple people familiar with the case who spoke on the condition of anonymity to candidly discuss an ongoing probe, investigators have continued to re-interview witnesses — including those who agreed to cooperate with the government as part of their plea deals.

Those interviews, the people said, appear aimed at firming up the prosecution’s case before asking a grand jury to return an indictment.

In the late summer, investigators were trained on actions the Gray administration may have taken to benefit Thompson, according to several people familiar with the case. Thompson once owned the company holding the city’s largest contract. The government appeared to be building a conspiracy case, alleging that Thompson supported Gray in exchange for favorable treatment for his firm, the individuals said.

Should Gray be indicted, he would be the first person targeted in the slow-moving campaign investigation not to agree to a plea deal. Gray’s defense team, according to people familiar with the case, believes the prosecution’s case would rest heavily on the credibility of Thompson — which they said could be discredited at trial by noting his favorable plea deal and admitted history of political corruption.

One lawyer involved in the case but not representing Gray said prosecutors would probably be able to secure an indictment — but would not face a simple task should the case go to trial. That may explain why Gray rejected a deal, the lawyer said.

“If we were where [Gray] was, you’d have to try me,” the lawyer said.

The prospect of an outgoing government executive facing federal indictment is reminiscent of the situation facing former Virginia governor Robert F. McDonnell (R) this past winter.

McDonnell’s indictment came in mid-January, less than two weeks after he had attended the inauguration of successor Terry McAuliffe (D) and after weeks of meetings between his legal team and federal prosecutors in Virginia. That timetable ultimately spared McDonnell the embarrassment of being charged while in office. After being informed that the U.S. attorney intended to seek an indictment, McDonnell’s attorneys, according to Washington Post reports, made a final appeal to top Justice Department officials asking them to review the charging decision.

That review included a meeting with the second-ranking federal law enforcement official. The indictment did not come for more than a month after that.

The person familiar with the Gray plea talks said Bennett made an argument to the assistant U.S. attorney overseeing the probe, Michael Atkinson, for why Gray should not be charged. He also asked that if Gray is indicted, the charges are returned only after Gray leaves office on Jan. 2 in order to provide a smooth transition for Bowser. There is no indication that Bennett has taken his entreaties to more senior officials.

In the McDonnell case, prosecutors in the Eastern District of Virginia reportedly told McDonnell’s defense team of their plans to seek an indictment but gave no indication of the precise timing.

Asked about the timing of potential charges against Gray, Machen said in March, “I don’t feel there’s a timetable. My obligation is to move forward when there’s enough evidence to move forward.”