By Brendan O'Brien
MILWAUKEE (Reuters) - A federal judge on Thursday halted an investigation into possible campaign finance law violations by conservative groups in Wisconsin, the second time he has done so this week.
U.S. District Court Judge Rudolph Randa said in a written decision that investigators issued a "frivolous" appeal when they said they had immunity from being sued.
Randa's ruling comes less than a day after a federal appeals court panel had ruled that the 20-month so-called John Doe probe could continue, allowing prosecutors legal room to call witnesses, request search warrants and offer immunity without probable cause that a crime has been committed.
The judge said Randa should not have entered a preliminary injunction because he knew investigators were planning to appeal.
The "John Doe" investigators could not be reached immediately for comment on Randa's Thursday ruling.
Randa on Tuesday ordered a halt to the investigation and said records and property seized by investigators must be returned to the Wisconsin Club for Growth, which had sued investigators, and any copies of the records destroyed.
He also said the club and its director were relieved of any and all duty to cooperate with the secret investigation.
The club sued investigators running the probe in February, accusing them of violating freedom of speech, association and equal protection rights by sidelining them from political activities during the 2014 election cycle that includes Republican Governor Scott Walker's re-election campaign.
The club said the investigations into its activities were politically driven, noting that they were initiated by Milwaukee County District Attorney John Chisholm, a Democrat.
The conservative lobby group said it had been targeted for alleged unlawful "coordination" with Walker's recall election campaign in 2012.
Democrats forced Walker to defend his seat in a special election in 2012. Armed with a massive fundraising effort spurred by conservatives inside and outside of Wisconsin, Walker won, becoming the first U.S. governor to survive a recall.
(Reporting by Brendan O'Brien; Editing by Richard Chang)