Updated

Dozens of conservative groups that support Wisconsin Republican Gov. Scott Walker reportedly have been subpoenaed by a special prosecutor demanding donor lists and other documents pertaining to their backing of Walker's union overhaul and recall fight.

The so-called "John Doe" investigation bars those subpoenaed from talking publicly.

But Eric O'Keefe, director of the Wisconsin Club for Growth, told The Wall Street Journal recently that investigators have raided at least three homes and that he "wants the public to know what is going on," despite the personal risk.

He also suggested the probe is having a chilling effect on conservative groups as Walker approaches a 2014 re-election effort.

He said the subpoenas, including the one he received in early October, "froze my communications and frightened many allies and vendors of the pro-taxpayer political movement in Wisconsin. ... The process is the punishment."

Watchdog.org reported in late October that authorities were confiscating equipment and files from targets of the probe, and demanding phone and email records. Watchdog.org also reported this week that three of the unidentified targets have hired top First Amendment and campaign finance experts as part of their defense team.

The Journal piece states more than two dozen groups received subpoenas -- ranging from the Walker campaign and state-level organizations such as O'Keefe's and Wisconsin Family Action to the national Republican Governors Association and American Crossroads, co-founded by former Bush administration adviser Karl Rove.

This is the second time in less than four years that Walker has been investigated.

In 2010, Milwaukee County Democratic District Attorney John Chisholm investigated whether staffers for Walker, when he was county executive, used government offices for political purposes. The probe closed in February with findings that included an aide sending campaign email on government time but no charges against Walker.

Both probes were first reported by The Milwaukee Journal Sentinel.

The new investigation purportedly asks for donor information from nonprofit groups not required to reveal such information and follows revelations this spring that conservative-leaning groups seeking tax-exempt status from the IRS were also targeted for extra scrutiny.

It began in the office of Milwaukee County Assistant District Attorney Bruce Landgraf, according to the Journal, and is being led by special prosecutor Francis Schmitz. Landgraf did not return a call this week seeking comment, and Schmitz also could not be reached.

The author of the Journal article says that he has seen copies of two subpoenas related to the 2011-12 recall effort on Walker and state senators, and that one demands: "all records of income received, including fundraising information and the identity of persons contributing to the corporation."

He writes the subpoenas don't make clear a specific allegation but the demands suggest the government is looking at the possibility of independent groups illegally coordinating with candidate campaigns.