A United States grand jury has decided not to bring charges against a white policeman over the killing of a 12-year-old black boy armed with a toy gun.
Ohio state prosecutor Tim McGinty called the events that led to the death of Tamir Rice a "perfect storm of human error".
But he said it was not unreasonable for the officer to fear for his life.
Public officials in Cleveland on Monday urged the public to remain calm and to protest peacefully.
State Senator Sandra Williams said any unrest would hamper progress but still called the decision a "grave miscarriage of justice".
The announcement comes at a time when the deaths of black men at the hands of police have sparked a national debate.
Rice was carrying a non-lethal pellet gun when police approached him in Cleveland in November 2014, in response to a 911 call reporting a man waving and pointing a gun at people.
The caller said the gun might not be real and the perpetrator could be a juvenile.
Mr McGinty, who announced the grand jury's decision on Monday, faulted the emergency services dispatcher for not relaying that information to police.
Officers Timothy Loehmann and Frank Garmback have said the gun looked real and urged Rice to raise his hands three times.
Mr Loehmann shot Rice twice after the boy pulled the gun from his waistband.
The officers said they believed Rice was older than 12. He weighed about 79kg and stood 1.7m tall.
The toy gun lacked an orange safety tip and Mr McGinty urged toy manufacturers to stop making replicas that look like real guns.
The Rice family have said police fired too quickly and should have used a Taser, a non-lethal weapon.
Footage from a surveillance camera shows Mr Loehmann firing moments after police arrived at the scene.
Although the officers' actions were not criminal, Mr McGinty said Cleveland had learned from the shooting.
"It should never happen again, and the city has taken steps so it doesn't,'' Mr McGinty said.