Among all the stresses and worries weighing on DC mayor Vincent Gray, the fact that the District’s top prosecutor, US Attorney Ronald Machen, is zeroing in on him must loom large.

DC’s government scandals are infamous: Marion Barry on camera with his crack pipe, bribery at the DC Taxicab Commission, a $48-million scam in the tax office. Yet the ongoing corruption probe into Gray’s 2010 campaign—coming on the heels of criminal investigations that took down two DC Council members—has rocked Washington like never before.

Machen, as the politically appointed head of the federal prosecutor’s office, doesn’t need to dig into the minutiae of cases. That can be left to lower-ranking prosecutors. But when asked if he’s personally involved in the Gray investigation, Machen smiles.

Seated in an oversize leather armchair at one end of his massive office overlooking Fourth Street, Northwest, he says that yes, he is very involved, then laughs at his own understatement.

Three of Mayor Vince Gray’s campaign aides have already accepted guilty pleas in Machen's investigation.

Before Machen can continue, his spokesman, Bill Miller, interrupts to remind him that technically he’s not supposed to use Gray’s name because the mayor has yet to be charged with any crimes and hasn’t been identified in court documents. “We call it the Candidate A investigation,” Miller says.

But if the identity of Candidate A is supposed to be a secret, it’s the worst-kept one in Washington. So far, Machen and his team of prosecutors have gotten guilty pleas from three of Gray’s campaign aides, and it seems only a matter of time before more members of the mayor’s inner circle—perhaps even the mayor himself—will fall.

Machen and Gray are caught in a high-stakes game of chicken: Will Gray give in to calls for his resignation first? Or will Machen and his investigators beat him to the punch, dealing a blow so incriminating that Gray is, at best, forced out of office, at worst, into a courtroom?

There’s no other job in the country like Machen’s. He’s one of the nation’s 94 US Attorneys, but as US Attorney for the District of Columbia, he has a uniquely diverse portfolio. He handles the sophisticated cases typical of a federal prosecutor—national-security matters, complex securities frauds, public corruption—but his office serves a dual purpose. Because DC doesn’t have an elected district attorney to go after its street criminals, Machen also oversees the prosecution of drug offenses, murders, rapes, and other crimes.

Asked what his priorities were when he became US Attorney, Machen lists public corruption first. From the beginning of his tenure, he wanted to emphasize holding officials accountable. If citizens can’t trust their government, he says, it weakens the democratic system: “The people of this city deserve ethical, strong leadership. And when people violate the law who are in positions of trust, there are going to be consequences.”

It’s hard to doubt him. Even as his office deals with budget cuts and a hiring freeze, he has aggressively allocated resources to investigations of DC officials, expanding his fraud and public-corruption unit and creating a special team of a dozen prosecutors who collaborate on the DC corruption cases. Machen often sits in on the team’s meetings, and he has assigned his principal assistant US Attorney, the office’s second-in-command, to lead it.

Aside from the Gray investigation, Machen’s office—the country’s largest US Attorney’s Office, with more than 350 lawyers and a budget of nearly $73 million—has secured guilty pleas in the recent corruption investigations of DC Council members Kwame Brown and Harry Thomas Jr.

Machen is living up to the reputation he’s built over the course of his career—first as a young prosecutor in the US Attorney’s Office in the ’90s, then as a partner in the white-collar criminal-defense practice at the high-end firm Wilmer, Cutler & Pickering (which later became WilmerHale)—as a dogged investigator with an eye for even the smallest holes in a case.

By Machen’s own admission, he can be a micromanager. He talks through trial strategies with prosecutors. He asks about key witnesses, meets with defense attorneys, and in the DC corruption cases has been involved in plea deals. Machen could leave these details to his line prosecutors—the lawyers who investigate and try cases—as many of his predecessors have, but that’s not how he likes to do things.

In addition to talking tough, Machen looks tough. With his six-foot-one frame, broad chest, and thick neck, the 43-year-old former Stanford wide receiver could be mistaken for a retired pro athlete. He wears a sharp suit, wingtips, and a goatee. He has a pierced left ear, and the diamonds in his wedding band occasionally glint as he gestures to emphasize a point.

The bags under his eyes hint at the grueling hours and stress of his job, but he doesn’t sound fatigued: “I get up, and I’m charged to come in here every day.”

His work, after all, isn’t done.