The Wisconsin Supreme Court refused Wednesday to reconsider its decision to end the "John Doe" investigation into Republican Gov. Scott Walker's recall campaign.

The state Government Accountability Board and Milwaukee prosecutors launched the investigation in 2012, to look into whether Walker's campaign illegally coordinated with outside conservative groups on issue ads. But in a major victory for the governor, the Supreme Court ended the probe in July, ruling that such coordination was permissible under state law.

So-called John Doe investigations are similar to grand jury proceedings. Information is kept secret and prosecutors can compel witnesses to testify as they weigh whether to pursue criminal charges.

No charges were filed in the Walker investigation, though it led to unsavory headlines about the governor. The conservative-leaning Supreme Court ordered prosecutors to return all evidence they'd collected during the probe and destroy any copies. Francis Schmitz, the special prosecutor who led the investigation, asked the high court in August to reconsider its decision.

Many of the court documents in the case -- including Schmitz's motion asking the court to reconsider its decision -- have never been released due to the John Doe secrecy rules.

The investigation focused on issue ads, which are campaign advertisements that advocate for or against a candidate's policies without expressly calling for his or her election or defeat. The Supreme Court ruled in July that the state's ban on campaigns coordinating with outside groups was so vague that it infringed on free-speech rights. The justices said the coordination ban applied only to express advocacy, which are ads that directly call for a named candidate's election or defeat.