CHICAGO — A police officer here who fatally shot an innocent bystander and a 19-year-old wielding a baseball bat will not face state charges, prosecutors announced on Friday.

The decision brought an end to a case that raised questions about training and the use of force by Chicago police, and one that came at the height of public outcry about other police shootings here.

The Cook County state’s attorney’s office said it found insufficient evidence to prove that Robert Rialmo, the police officer, was not legally justified in the shooting in the early-morning hours of Dec. 26, 2015, at a home on this city’s West Side.

Officer Rialmo was responding to 911 calls from the home when he shot and killed Quintonio LeGrier, a college student, as well as a neighbor, Bettie Jones, who had merely answered the front door for the police.