LOS ANGELES (NYTIMES) - Two Sacramento police officers who shot and killed an unarmed black man in his grandmother's backyard last year will not face criminal prosecution, the Sacramento County district attorney has announced, stirring fresh outrage in a city roiled by protests over the killing.

For nearly a year, community members and activists have demanded police accountability for the death of Mr Stephon Clark, 22, who was killed in March last year by officers Terrence Mercadal and Jared Robinet.

The officers had been dispatched to investigate a vandalism complaint. Within 10 minutes of their arrival, after a brief pursuit, Mr Clark was dead.

Mr Clark's death took on national significance amid continuing tensions over discriminatory policing in black neighbourhoods and excessive use of force by officers.

"Was a crime committed? There's no question that a human being died," District Attorney Anne Marie Schubert said on Saturday (March 2) in Sacramento. "But when we look at the facts and the law, and we follow our ethical responsibilities, the answer to that question is no. And as a result, we will not charge these officers."

She said the officers had probable cause to stop and detain Mr Clark. She added that police officers are legally justified in using deadly force "if the officer honestly and reasonably believes" he is in danger of death or injury.

"We must recognise that they are often forced to make split-second decisions," she said. "We must also recognise that they are under tense, uncertain and rapidly evolving circumstances."

The officers fired their weapons 20 times within seconds of turning a blind corner.

"Both officers believed that he was pointing a gun at them," Ms Schubert said. She added that police video showed Mr Clark was "advancing" on the officers.

Mr Clark was later found to be unarmed; his mobile phone was found under his body.

The police department released body camera footage within days of the shooting. Mr Daniel Hahn, the city's first black police chief, reiterated that the department required training related to race-based discrimination and de-escalation tactics.

During a news conference on Saturday, Mr Clark's mother, Ms Sequette Clark, expressed outrage at the decision not to prosecute. She blasted Ms Schubert for delving into details she said were irrelevant to the officers' conduct, including personal text messages and a toxicology report showing that her son had alcohol, codeine, marijuana, cocaine and Xanax in his system.