CLEVELAND — A grand jury declined on Monday to charge a Cleveland patrolman who fatally shot a 12-year-old boy holding a pellet gun, capping more than a year of investigation into a case that added to national outrage over white officers killing African-Americans.

In announcing the decision, Timothy J. McGinty, the Cuyahoga County prosecutor, said he had recommended that the grand jurors not bring charges in the killing of the boy, Tamir Rice, who was playing with the gun outside a recreation center in November 2014.

Mr. McGinty said the fatal encounter had been a tragedy and a “perfect storm of human error, mistakes and miscommunications.” But he said that enhancement of video from the scene had made it “indisputable” that Tamir, who was black, was drawing the pellet gun from his waistband when he was shot, either to hand it over to the officers or to show them that it was not a real firearm. He said that there was no reason for the officers to know that, and that the officer who fired, Timothy Loehmann, had a reason to fear for his life.

The case began when a caller to 911 said a male was pointing a gun at people in a Cleveland park. The caller added that the gun was “probably fake,” and that the person waving it was “probably a juvenile.” But those caveats were not relayed to Officer Loehmann or his partner, Frank Garmback, who was driving the patrol car. Officer Loehmann, who is white, opened fire within seconds of arriving at the park. Officer Garmback was also spared any charges.