Missouri’s highest court says it won’t intervene in a lawsuit by activists seeking an independent probe of a prosecutor’s handling of grand jury proceedings in the Ferguson police shooting of Michael Brown.

The Missouri Supreme Court announced in an order obtained by media outlets Friday that it won’t hear the matter.

Activists were challenging a state appellate court’s May conclusion that a St. Louis County judge correctly dismissed the suit, which sought a special prosecutor to scrutinize prosecutor Robert McCulloch’s conduct during the secret grand jury proceedings.

The activists questioned McCulloch’s role. The grand jury cleared white Ferguson police officer Darren Wilson in the fatal 2014 shooting of 18-year-old Brown, who was black and unarmed. Wilson later resigned.

A U.S. Justice Department probe concluded Wilson acted in self-defense.