The spokesman, Cullen Werwie, also served as deputy communications director for Walker's gubernatorial campaign.
Rose Ann Dieck, a retired teacher and Milwaukee County Republican party activist, and Kenneth Lucht, a lobbyist for the Wisconsin & Southern Railroad, have also been granted immunity in matters "still under inquiry" through the secret probe, according to the judge overseeing the case.
The judge stressed that a grant of immunity "does not necessarily mean, imply or infer that those witnesses are suspected of, or guilty of, any criminal wrongdoing."
WisPolitics also notes that Lucht, the lobbyist who has been granted immunity, was previously ordered in April to pay a $250 civil forfeiture, connected with the investigation.
The site further reports that Werwie has no comment on the matter -- which makes sense, given the secretive nature of the investigation.
The "John Doe" investigation -- a secret proceeding in which witnesses can be subpoenaed to testify under oath, but are forbidden from talking publicly about the case -- is reported to have originally stemmed from a staffer resigning in 2010, when she was found using her county time to post reader comments on online newspaper article promoting Walker's gubernatorial candidacy and criticizing his opponents.
Last week, former Walker aide Cynthia Archer's home was raided by the FBI, though she has denied any connection with the John Doe investigation.