WTOP’s Mark Plotkin did a public service Friday morning by inviting U.S. Attorney Ronald C. Machen Jr. onto his program and trying his darnedest to get him to talk about his office’s investigations into city government.

As you might expect, there was a lot of “I can’t answer that” coming out of the top prosecutor’s mouth.

But Machen did discuss the importance of pursuing public corruption — “It’s important that people understand no one is above the law, and public officials have to be held to a very high standard” — and did specifically acknowledge that his office is investigating Mayor Vincent C. Gray’s campaign and D.C. Council member Harry Thomas Jr. (D-Ward 5).

Yes, that news has already been disclosed in written statements, but there is something reassuring about hearing it come out of Machen’s mouth. The comments, if nothing else, reinforce that he’s personally engaged in the issue and that the allegations won’t fall into a “black hole” (which is how civic activist Dorothy Brizill described the U.S. attorney’s office in a call to the show).

To put a real fine point on it, Plotkin asked Machen, to no avail, if he has interviewed Sulaimon Brown, Lorraine Green and Howard Brooks — all key figures in the Gray probe.

Plotkin also asked about the taxi corruption case that ensnared Jim Graham’s chief of staff. Plotkin pressed Machen on the revelation that Graham (D-Ward 1) had rejected a bribe, but did not report it.

Machen demurred on specifics, natch, but he did offer this sage advice: “Any time somebody’s giving you cash, you’ve got to ask yourself: Why are they giving me cash; what are they expecting me to do?” he said. “If you think somebody is trying to bribe you, you should report that.”

The number, incidentally, is (202) 252-7566.

Perhaps the larger public service performed by Plotkin, however, was debunking the myth of the “grand referral.” In this town, it has become a big deal if local authorities refer matters to the feds for prosecution.

But the truth, Machen made clear, is that an official referral doesn’t amount to a whole lot. Federal prosecutors read the paper and watch TV news, too, and they can smell when something’s fishy. They don’t usually need the city’s attorney general or the D.C. Council or the Office of Campaign Finance to sniff for them.

”A referral, it doesn’t mean we’re not looking at something until we get a referral,” Machen said. “It doesn’t matter who refers an allegation to us, we’re going to look into it.”

Once again, (202) 252-7566.