When people in Washington dismiss complaints about a special counsel’s “witch hunt” investigation with broad powers and no accountability, they should consider what happened in Wisconsin.

Wisconsin’s attorney general, a Republican, is recommending disciplinary action but no criminal charges against a former Democratic appointee and other state officials over the leak of sealed evidence in a secret investigation that invaded the privacy and seized emails of dozens of Republican officials allied with Gov. Scott Walker, including former White House Chief of Staff Reince Priebus, Virginia gubernatorial candidate Ed Gillespie, Sen. Ron Johnson of Wisconsin, another Senate candidate and conservative groups such as the Federalist Society and the Wisconsin Club for Growth.

In a 91-page report released this week, state Attorney General Brad Schimel revealed the actions of staff at the now-defunct state Government Accountability Board and a “John Doe III” investigation into Republican officials and staffers who were suspected of campaigning out of taxpayer-funded offices.

The board’s investigation of state Republican officials found no wrongdoing. But a leak to the Guardian newspaper of information from the “John Doe” investigations, which were authorized under state law, prompted a state Justice Department review that uncovered “a previously unknown and secret investigation into a broad range of Wisconsin Republicans.”

That review showed that investigators had collected thousands of personal emails of at least 35 Republicans, issued 218 warrants and subpoenas, seized personal property of conservatives and conducted paramilitary-style pre-dawn raids of suspects’ homes with battering rams and floodlights.

One woman whose front door was battered in while she was sleeping thought she was a victim of a criminal’s home invasion.

Milwaukee County District Attorney John Chisholm and “John Doe” special prosecutor John Schmitz controlled the investigation into whether Mr. Walker’s campaign illegally coordinated with conservative groups in recall elections. Mr. Chisholm is a Democrat; Mr. Schmitz has identified himself as a Republican.

The state Supreme Court closed the investigation in 2015 after finding no wrongdoing. But with the release of the state Justice Department’s review of the secretive operations this week, the full extent of the investigators’ abuse of power has become clearer.

State Sen. Leah Vukmir, Brookfield Republican, learned that government investigators had collected her emails as far back as 2009 because she was a target of the probe. Emails with her daughter included private medical information and other personal details. State investigators kept those emails in a file labeled “opposition research.”

Mrs. Vukmir blasted the “witch hunt” by Mr. Chisholm and the disbanded Government Accountability Board, saying the invasion of her privacy is “absolutely appalling.”

“This criminal behavior is inexcusable, and the individuals involved in this belong in prison,” Mrs. Vukmir said. “I am horrified to learn again just how invasive this witch hunt was into nearly every part of every major Wisconsin conservative’s life. My privacy has been violated in such a brazen way by those who targeted me, my friends and even my family. No political motivation justifies this. The thought of my government intentionally and illegally targeting me is sickening.”

Mrs. Vukmir is now a candidate for the U.S. Senate, seeking to unseat Democrat Tammy Baldwin next year.

Mr. Schimel’s probe found that government investigators obtained the personal email accounts and IP login information for 35 Republican aides and party operatives, as well as hundreds of thousands of private emails they exchanged with Mr. Walker, Assembly Speaker Robin Vos, Mr. Johnson, former U.S. Senate candidate Terrence Wall, Mr. Priebus, Rep. Sean P. Duffy, and several other state legislators.

Mr. Priebus could not be reached for comment Thursday.

Among the emails collected that had nothing to do with campaign finance laws were more than 1,000 messages between a private Bible study group, a string of 20 exchanges referencing a “Kenmore Mini Fridge” negotiation over Craigslist, an application for a bank mortgage and a discussion between parents discussing their daughter’s need for an obstetrician.

According to the state Department of Justice report, items seized as evidence in the sweeping investigation included three hard drives containing nearly 500,000 unique emails and other documents such as email attachments totaling millions of pages.

“The hard drives included transcripts of Google Chat logs between several individuals, most of which were purely personal (and sometimes very private) conversations,” the report said. “GAB placed a large portion of these emails into several folders entitled ‘Opposition Research’ or ‘Senate Opposition Research.’ [The state Justice Department] is deeply concerned by what appears to have been the weaponizing of GAB by partisans in furtherance of political goals. Indeed, it is difficult to conceive why GAB needed any information from GoDaddy.com related to former Republican Senate Leadership Association Chairman Ed Gillespie or why staff attorneys wanted information held by Google for Leonard Leo, executive director of the Federalist Society.”

Mr. Leo could not be reached for comment.

The attorney general is recommending contempt proceedings against the special prosecutor, Mr. Schmitz, and six others on his team for violating court orders related to the handling of seized evidence, plus referring former GAB attorney Shane Falk to the state judiciary’s office of lawyer regulation for discipline.

“The systemic and pervasive mishandling of John Doe evidence likely resulted in circumstances allowing the Guardian leak in the first place, and now prevents prosecutors from proving criminal liability beyond a reasonable doubt,” the report said.

Mr. Schimel’s investigation was prompted by the leak of documents to the Guardian, which published an article shortly before the U.S. Supreme Court decided not to review the Wisconsin Supreme Court’s decision to shutter the second “John Doe” investigation.

The first “John Doe” investigation, which looked into Mr. Walker’s Milwaukee County office, obtained six convictions, including two defendants for campaigning on taxpayer time. That led to a second investigation, within a month of Mr. Walker’s recall election victory in 2012, that focused on the governor’s fundraising activities.

The district attorney’s office began with working with the board, which incorrectly informed investigators that conservative groups had illegally coordinated their activities during the recall election.

A judge ordered the investigative team in January 2014 not to examine property and evidence seized in the case. But the state Justice Department’s report said Mr. Falk disregarded the order and directed the team to compile records of donations to and from the Wisconsin Club for Growth and to compile records from a database containing emails seized through the search warrants.

The special prosecutor found out about Mr. Falk’s actions but failed to stop him.

The Wisconsin Supreme Court ended the John Doe II investigation on July 16, 2015, “because the special prosecutor’s legal theory [was] unsupported in either reason or law.” It ordered the special prosecutor and the district attorneys in the probe to “cease all activities related to the investigation, return all property seized in the investigation from any individual or organization, and permanently destroy all copies of information and other materials obtained through the investigation.”

But the prosecution team instead took the case to the U.S. Supreme Court, which scheduled a conference to consider the matter on Sept. 26, 2016. Eleven days before that conference, the Guardian published leaked documents, part of the 1,500 pages it had obtained on the John Doe II investigation.

The state Legislature responded to the “John Doe II” investigation by disbanding the board in 2016 and breaking it into two parts: the Wisconsin Ethics Commission and the Wisconsin Elections Commission.