MADISON, Wis. (AP) — The Wisconsin Supreme Court on Thursday ended a secret investigation into whether Republican Gov. Scott Walker's campaign illegally coordinated with conservative groups in winning his 2012 recall election.

The court's action is a major victory for Walker as he pursues the presidency, though one expert said prosecutors could appeal to the U.S. Supreme Court. No one has been charged in the so-called John Doe probe, Wisconsin's version of a grand jury investigation where information is tightly controlled, but questions about the investigation have dogged Walker for months.

The case centers on political activity conducted by Wisconsin Club for Growth and other conservative organizations during the 2012 recall, which was spurred by Democrats' anger over a Walker-authored law that effectively ending collective bargaining for most public workers.

Prosecutors accused Walker and the groups of illegally coordinating their campaign efforts in violation of state law. They denied wrongdoing.

Republicans called the investigation, launched by Milwaukee County District Attorney John Chisholm, a Democrat, a partisan witch hunt. Wisconsin Club for Growth and its director, Eric O'Keefe, filed a federal civil rights lawsuit last year seeking to halt the probe, arguing the investigation violates their free speech rights. U.S. District Judge Rudolph Randa sided with the club but a federal appellate court later tossed out the lawsuit, saying the issue belonged in state courts.

The club and O'Keefe then turned to the state Supreme Court, which is controlled by a four-justice conservative majority.

Howard Schweber, an associate professor of political science and legal studies at the University of Wisconsin-Madison, said prosecutors could seek review by the U.S. Supreme Court, possibly arguing bias. Lead prosecutor Francis Schmitz in February asked at least two justices to recuse themselves to avoid the appearance of impropriety, but the justices didn't respond.