A grand jury has declined to indict a white rookie police officer in the killing of 12-year-old Tamir Rice, a black youngster shot to death while playing with what turned out to be a pellet gun.
Prosecutor Tim McGinty said it was "indisputable" the boy was drawing the pistol from his waistband when he was gunned down - either to hand it over to police or show them that it was not real.
However, Mr McGinty said there was no way for the officers on the scene to know that.
"Simply put, given this perfect storm of human error, mistakes and miscommunications by all involved that day, the evidence did not indicate criminal conduct by police," he said.
Mr McGinty said patrolman Timothy Loehmann was justified in opening fire: "He had reason to fear for his life."
Tamir was shot by Loehmann within two seconds of the officer's police cruiser skidding to a stop near the boy outside a city recreation centre in Cleveland, Ohio, in November 2014.
Loehmann and his training partner Frank Garmback were responding to an emergency call about a man waving a gun.
Tamir was carrying a borrowed airsoft gun that looked like a real gun but shot non-lethal plastic pellets. It was missing its tell-tale orange tip.
A grainy video of the shooting captured by a surveillance camera provoked outrage nationally and together with other killings of black people by police in places such as Ferguson, Missouri, and New York City, it helped fuel the Black Lives Matter movement.
Mr McGinty urged those who disagree with the grand jury decision to react peacefully and said: "It is time for the community and all of us to start to heal."
The grand jury had been hearing evidence and testimony since mid-October.
In explaining the decision not to bring charges, the prosecutor said police radio personnel contributed to the tragedy by failing to pass along the "all-important fact" the person who called police said the gunman was probably a juvenile and the gun probably was not real.
Assistant Prosecutor Matthew Meyer said it was "extremely difficult" to tell the difference between the fake gun and a real one since the orange tip had been removed.
He also said Tamir was big for his age - 5ft 7ins - and could have easily passed for someone older.
"There have been lessons learned already. It should never happen again and the city has taken steps so it doesn't," Mr McGinty said.
Among other things, the Cleveland police department is putting dashboard cameras in every car and equipping officers with bodycams.
The Cleveland police department also reached a settlement with the US Justice Department earlier this year to overhaul the way it uses force and deals with the public.
It was prompted in large part by the killing of a couple in a 137-shot barrage of police gunfire after a high-speed chase.
Mr McGinty said it had been a "tough conversation" with Tamir's mother when she was told there would be no charges. "She was broken up and it was very hard," the prosecutor said.
Tamir's family has filed a federal civil rights lawsuit against the two officers and the city of Cleveland.