WASHINGTON (AP) — The Supreme Court has rejected an appeal to reopen an investigation into whether Wisconsin Gov. Scott Walker’s campaign to fend off his ouster from office illegally coordinated with outside conservative groups.

The justices on Monday left in place a Wisconsin Supreme Court decision that shut down the probe of the Republican governor. Walker won his recall election in 2012.

A group of prosecutors began an investigation the same year into whether Walker’s campaign coordinated with Wisconsin Club for Growth and other conservative groups on advertising during the recall without reporting the groups’ contributions.

Those groups challenged the investigation. Conservative-leaning justices who control the Wisconsin Supreme Court halted it last year. They ruled that the groups and the campaign coordinated on ads that did not expressly call for a candidate’s election or defeat. The state court said such coordination amounts to free speech and isn’t subject to disclosure requirements.

The controversy attracted renewed attention recently when The Guardian newspaper obtained more than 1,000 pages of leaked documents from the investigation. The newspaper reported that a leading manufacturer of lead that was once used in paint was among a host of corporate leaders who donated to a conservative group that helped Walker and Republican legislators fight the recall challenges.

Wisconsin Club for Growth received $750,000 in 2011 and 2012 from Harold Simmons, the billionaire owner of NL Industries. The company produced lead that was used in paint before such practices were banned. Walker and his fundraisers also solicited money from hedge-fund billionaire Stephen Cohen, who gave the club $1 million; Home Depot co-founder Ken Langone, who gave $25,000; and hedge-fund manager and Manhattan Institute for Policy Research Chairman Paul Singer, who gave $250,000.

Walker has called the information contained in the leaked documents “old news” and insisted he did nothing wrong.

Walker declined immediate comment Monday through his spokesman Joe Fadness. The governor, who has been working to prepare Indiana Gov. Mike Pence for the vice presidential debate on Tuesday, had no public events planned.

The recall itself stemmed from Walker’s signature law that stripped public unions of nearly all of their bargaining rights.

The prosecutors are Milwaukee County District Attorney John Chisholm, Iowa County District Attorney Larry Nelson and Dane County District Attorney Ismael Ozanne — all Democrats. They urged the high court to decide how broadly free speech rights protect campaign coordination with outside groups. They also said that the judicial process was unfair because two state court justices had received campaign help from some of the groups under investigation and should have stepped aside. The justices are Michael Gableman and David Prosser, who has since left the court.

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