U.S Attorney Ronald C. Machen Jr., left, said at a forum at a Capitol Hill community center Wednesday night, Nov. 20, 2013, that some of the people urging him to finish his probe into Mayor Vincent C. Gray’s 2010 campaign are not cooperating with it. At right is WRC-TV reporter Tom Sherwood. (Aaron C. Davis /The Washington Post)

With his investigation into the last D.C. mayoral race now impinging on the next one, the District’s top prosecutor said “challenges and obstacles” have kept him from closing out his long-running probe into city political corruption.

U.S. Attorney Ronald C. Machen Jr., speaking at an event Wednesday evening, said the public should not conclude that the probe, centered on the 2010 campaign of Mayor Vincent C. Gray (D), has stalled indefinitely.

“You’ve got four people associated with a mayoral campaign who have pled guilty to felonies,” he said. “It’s not like we’ve been looking at this for three years and there’s no there there. I mean, there’s there there, and we’re trying to gather information, we’re trying to get documents, and we’re trying to talk to people.”

Machen’s comments, which came during a question-and-answer session at a Capitol Hill community center, were among the most extensive comments he has made on the nearly three-year-old investigation, but they offered little clarity on when new charges might come or whom they might involve.

Still, former federal prosecutors interviewed Thursday said it is rare for a U.S. attorney to describe an ongoing investigation in those terms.

Mayor Vincent Gray speaks to the media Tuesday after officiating a same-sex wedding in D.C. (Marlon Correa/The Washington Post)

“That is quite unusual,” said Jonathan Biran, a lawyer in Maryland who previously worked in the Justice Department’s public integrity unit. “He probably would not have made a statement like that unless he felt he had to respond and let folks know, to the extent he could, what the issues were.”

Questions about the course of Machen’s investigation have multiplied in recent weeks as the 2014 campaign cycle has progressed without a decision from Gray about whether he intends to seek another term.

Gray, who has not been charged with any crime, acknowledged in a Thursday television interview that the ongoing investigation has played a significant role in keeping him from announcing his plans.

“Why the hesitation?” NewsChannel 8 host Bruce DePuyt asked him.

“Obviously, Bruce, we’d love this situation to be over with,” Gray said. “It’s gone on for a very long time. And it would be better to have that behind me.”

Machen said Wednesday that he was well aware of the case’s high profile and its potential influence on the upcoming election. But the campaign calendar would not dictate his timetable for bringing charges, he said: “We can’t just wrap up an investigation when we’re still in the midst of gathering information.”

Mary Pat Brown, a D.C. lawyer who previously directed federal criminal prosecutions in the District, said Justice Department guidelines strictly limit prosecutors from influencing elections. But she said those rules are often misinterpreted.

“The public does not have the right to know all the dirt on either candidate before an election,” she said. Any investigation concurrent with an election campaign “has to be on a strictly separated path,” she said.

Among the obstacles delaying the probe, Machen said, is his unresolved dispute with D.C. Attorney General Irvin B. Nathan over access to documents relating to the city’s 2011 settlement of claims by a health-care firm owned by businessman Jeffrey E. Thompson.

Thompson, several people with knowledge of the investigation say, is the unnamed executive described in court documents as funding a secret $655,000 “shadow campaign” on Gray’s behalf. Nathan has said the documents Machen is seeking are protected by attorney-client privilege, and he has defended the settlement as aboveboard.

Machen’s investigation also has been slowed by a lengthy legal battle over documents seized from Thompson’s home and offices in March 2012. Thompson’s attorneys have argued in court filings that prosecutors are not entitled to review a subset of the records and have appealed to the U.S. Supreme Court.

The wrangling over Thompson’s records is unlikely to be resolved soon.

The Supreme Court will not decide whether to hear the appeal until after the government has responded to Thompson’s petition. Last week, the deadline for that response was extended until Dec. 20.

Machen on Wednesday did not acknowledge Thompson’s alleged role in the investigation or many other specifics when Tom Sherwood, a WRC (Channel 4) reporter, pressed him for details.

But the U.S. attorney pointedly said that “some of the same people who are saying, ‘Hurry up, hurry up,’ may also be involved in not coming forward with the information we need.”

Machen did not name any particular individuals, and Gray demurred Thursday when asked about the comments. “I think [Machen] would have to further explain that himself,” he said. “I don’t know who he’s referring to or what he’s referring to, frankly.”

Gray has denied engaging in any wrongdoing and has said the investigation should run its course. But he is under considerable pressure to announce his reelection decision as challengers, including four D.C. Council members, line up to run in the April 1 Democratic primary.

Gray said he hasn’t set an “ironclad date” for making a decision but acknowledged that it will become more difficult to gather the necessary voter signatures to appear on the April 1 primary ballot as an early January deadline approaches.

“I have no control over [the investigation],” he said. “I have no say in that. So, you know, I will have to make a decision based largely on the facts in front of me at this stage.”

Aaron C. Davis contributed to this report.