A secret Milwaukee County investigation that has already led to criminal charges against five people close to Gov. Scott Walker took a step forward Wednesday when a judge granted immunity to a 12th witness.

David Halbrooks, a former Milwaukee municipal judge and private attorney, requested and received immunity in the so-called John Doe investigation, according to online court records.

Halbrooks declined comment when reached by The Associated Press.

A John Doe investigation is one in which witnesses can be compelled to testify under oath about potential criminal matters. The case is conducted in secrecy, and the only part of a John Doe investigation that is public is the granting of immunity. The proceeding allows witnesses to be subpoenaed, while forbidding them to talk publicly about the case.

Eleven other people, including Walker spokesman Cullen Werwie, were granted immunity between July 2010 and April 2011.

The secret investigation centers on people who served during Walker's time as the Milwaukee County executive before he was elected governor in 2010. Allegations ranging from campaigning on county time to embezzling money from a veterans program have ensnared people close to Walker.

Walker himself hasn't been charged with wrongdoing and has repeatedly said he's cooperating with the investigation. However, he also set up a legal defense fund, a move allowed only for officeholders who have been charged or are under investigation for election or campaign violations.

Walker's campaign revealed in recent public filings that it transferred $60,000 to the legal fund to pay for attorneys.

The defense fund has already become a campaign issue for Walker. The Republican governor faces a recall election next month that was sparked when Walker and Republicans passed a law that effectively ended collective-bargaining rights for most public workers.

Milwaukee Mayor Tom Barrett, the Democrat challenging Walker in the June 5 election, called on the governor last week to reveal the circumstances under which he arranged the legal defense fund.

Walker's campaign responded that the fund was set up under the guidance of the state elections board and reiterated that Walker isn't a target of the investigation.

The ongoing John Doe case has already led to charges against five people who worked for or were associated with Walker's county executive office in Milwaukee before he was became governor.

One former aide pleaded guilty to working on Walker's gubernatorial campaign on county time, and his former deputy chief of staff is charged with felony misconduct in office. Two other former Walker associates were charged with embezzling more than $60,000 from veterans and their families, and a fifth is charged with child enticement, evidence of which allegedly was discovered while investigating one of the others.