It’s a topic so hot in Wisconsin, no one involved with it can talk about it. A wide-ranging investigation, a special prosecutor, and the cloak of secrecy.

The Milwaukee Journal Sentinel reported this week that a former federal prosecutor had been appointed to lead a “John Doe investigation” into state-level issues in Wisconsin, including the 2011 and 2012 recall elections. And this John Doe investigation reportedly grew out of an earlier John Doe probe, which closed in March after snaring six former aides and associates of Gov. Scott Walker (R).

But hang on a second. What exactly is a John Doe investigation? For those outside Wisconsin, the term may be unfamiliar, and with good reason. Wisconsin may be the only state that uses them.

David Schultz, who teaches criminal law at the University of Wisconsin Law School, spoke to TPM on Wednesday, and explained how John Doe investigations work and why they’re used.

“The most common description of them is, it’s just like a grand jury except no jury,” Schultz said. “It’s a secret proceeding [in which] you can require people to give testimony and, as I say, therefore obtain information that you might not be able to get with just voluntary cooperation.”

According to Schultz, grand juries are rarely used in Wisconsin, in large part because “this more compact, accessible John Doe procedure is available.”

John Doe proceedings seek to determine if there is a basis to charge a criminal offense. But unlike a grand jury, there is no panel of jurors in a John Doe investigation. There is just a judge. And although district attorneys do not need to clear much of a bar to request a John Doe proceeding, Schultz said that, in practice, judges expect some basis to justify the proceeding.

“Although [John Doe proceedings] are not unheard of or unusual, they’re used seldom enough that I expect that a judge is going to require some significant showing, to make sure that this isn’t just a fishing expedition of some sort,” Schultz said. “And there has to be some basis before this procedure is started up.”

In practice, John Doe proceedings will also commonly offer immunity to people in exchange for testimony.

“In order to get the evidence, you give immunity to some people, ostensibly to move higher up the ladder, I suppose,” Schultz said.

And while John Doe proceedings do not need to be secret, they almost always are.

“Each person involved, if it is a secret proceeding, is told that it’s secret, and ordered not to talk about it,” Schultz said. “By involved, I mean someone who has been called to testify as part of the proceeding.”

That fact helps explain the response Walker gave on Monday to questions about the newly revealed investigation. While he called the overall matter a “sidebar issue,” he also alluded to secrecy rules as a reason he wouldn’t say whether he, his attorneys, or any of his staff members have been contacted in the ongoing John Doe investigation.

“You know the circumstances of that, I’m not going to get pulled into that one way or the other,” Walker told reporter, according to video of the event put online by the WisconsinEye Network. “Because if somebody was, and they said they were, they’d be in violation of that. If they weren’t, they’d be speculating on something that they’re not involved in. So either way, it doesn’t make sense to be involved in that.”

According to the Journal Sentinel, the current investigation opened in February 2012, involves multiple Wisconsin counties, and is looking at a number of issues, including the recall races, a current legislative leader, and the 2012 gubernatorial recall contest between Walker and Milwaukee Mayor Tom Barrett. Francis Schmitz, a former assistant U.S. attorney, has reportedly been appointed as a special prosecutor to lead the probe.

The previous John Doe investigation, which closed in March, uncovered misdeeds including illegal campaign contributions, illegal campaign activity, and embezzlement of veterans’ funds from Walker’s time as Milwaukee County executive, according to The Wisconsin State Journal. All told, six people plead to charges ranging from stealing tens of thousands of dollars from a veterans event to doing campaign work while on the county clock, according to the Journal Sentinel.