The top prosecutor in Minneapolis has dropped murder charges against a state trooper who fatally shot a motorist last year after a traffic stop, her office said on Sunday, a stunning turnaround in a case that ignited a political firestorm.

The trooper, Ryan Londregan, had been charged with second-degree murder in the killing of Ricky Cobb II. But the prosecutor, Mary Moriarty, a longtime public defender who was elected Hennepin County attorney in 2022, said she concluded that the evidence was too weak to take to trial.

For months, Ms. Moriarty defended the murder charges amid criticism from both Democratic and Republican officials, as well as law enforcement officials. In a statement on Sunday, she said that the announcement dismissing the charges was “one of the most difficult I’ve made in my career.”

The pushback over the charges reflected a shifting view on policing in the state four years after the murder of George Floyd by a Minneapolis police officer sparked a national outcry over racism and abuses by law enforcement. Mr. Cobb, 33, was Black; Trooper Londregan, 27, is white.

Ms. Moriarty took office promising sweeping changes in the wake of Mr. Floyd’s murder, including stronger efforts to hold officers accountable for misconduct. Civil rights activists had hailed her decision to charge Trooper Londregan as courageous.

Gov. Tim Walz, a fellow Democrat, had voiced his unease and made clear that he was considering using his legal authority to remove the case from her purview. In recent months, six of the state’s eight members of Congress issued statements criticizing the prosecution.