Chicago - A grand jury in the US state of Ohio declined to bring criminal charges against Cleveland police officers involved in the fatal shooting last year of a 12-year-old boy, a prosecutor said Monday.The November 2014 death of Tamir Rice – a black child who had been carrying a replica gun in a playground when he was shot dead – and the fatal shootings of other African Americans by police have triggered protests across the country.Surveillance video showed Rice was fatally shot within seconds of the patrol car arriving on the scene as he began to pull the toy gun out of his waistband. The boy died hours later in hospital.Cuyahoga County prosecutor Tim McGinty described a "perfect storm of human error, mistakes and communications by all involved that day" – and said evidence considered by the grand jury "did not indicate criminal conduct by police.""It would be irresponsible and unreasonable if the law required a police officer to wait and see if the gun was real," McGinty told reporters.The Rice shooting came just days before a grand jury opted not to indict a white police officer who fatally shot unarmed black teenager Michael Brown in the St Louis, Missouri, suburb of Ferguson in August 2014.The two incidents are frequently cited in the ongoing national debate about how race plays into police actions in the United States.In the latest incident to raise hackles, police in Chicago responding to a domestic dispute on Sunday shot dead a young black man who was allegedly holding a baseball bat as he came down the stairs and also killed his neighbour, a mother of five who had answered the door.Saddened, not surprisedRice's family said they were "saddened and disappointed" by the grand jury's decision "but not surprised."They accused McGinty of "abusing and manipulating the grand jury process to orchestrate a vote against indictment" and urged federal prosecutors to "step in to conduct a real investigation."Ohio Governor John Kasich urged residents not to "give in to anger and frustration and let it divide us.""Tamir Rice's death was a heartbreaking tragedy and I understand how this decision will leave many people asking themselves if justice was served," he said in a statement."We have made progress to improve the way communities and police work together in our state, and we're beginning to see a path to positive change so everyone shares in the safety and success they deserve."A judge had recommended in June that there was probable cause to charge the officers, but independent reports ordered by McGinty's office and released in October found that officer Timothy Loehmann was justified in shooting Rice.Loehmann and his partner Frank Garmback believed they were responding to an "active shooter" in a crime-ridden park that has memorials to two police officers who had been shot dead nearby in the line of duty, McGinty said on Monday.A police dispatcher had failed to tell them that the person who called to complain about Rice brandishing a gun had said he believed it was a toy.The officers were "frightened" and did not realise that Rice – who was tall for his age – was just a boy with a toy, McGinty said.Pattern of excessive forceIn December 2014, a federal probe launched by the Justice Department – well before the Rice shooting – found that Cleveland police had engaged in a pattern of using excessive force.Cleveland – a city of 390 000 that is more than 50% African-American – pledged in May to overhaul its police force and aspire to "bias-free" law enforcement, under an agreement with the Justice Department.McGinty insisted that "steps have been taken" to ensure that this "tragic event" does not happen again, including outfitting all Cleveland police officers with body cameras in order to help "improve public confidence and improve performance."Cleveland Mayor Frank Jackson acknowledged it had been a "long, troubling, trying year" for the city, and especially for Rice's family, and pledged that a police administrative review of the incident would move forward.Later on Monday, dozens of people protested in New York City, trying to block streets to express outrage at the Ohio grand jury decision in Rice's case.