Anger threatened to boil over in the US state of Minnesota on Friday after Jeronimo Yanez, the police officer who fatally shot a black motorist last year, was found not guilty.

The jury of seven men and five women, 10 of them white and two black, acquitted Yanez on all charges.

“We're very happy. Yanez was innocent. He was just doing his job,” said an attorney for the officer.

However, the family of victim Philando Castile was furious and pointed to racial prejudice in the case. 

“I’m mad as hell right now. My first-born son died… Just because he was a police officer, that makes it Ok,” said the victim’s mother. She said the verdict showed “the system continues to fail black people.”

Hundreds of demonstrators also gathered outside the state capitol in St. Paul on Friday evening, demanding justice for minorities and stronger accountability measures for police. 

Friday's verdict drew an angry reaction from Castile's mother, as she told reporters she was "mad as hell." /CCTV Photo

The shooting last July shocked the US after Castile’s girlfriend, Diamond Reynolds, recorded the aftermath of the brutal incident and livestreamed it on Facebook.

Castile had been shot five times in the arm after he informed Yanez he had a firearm when he was requested to pull over due to a broken taillight.

Castile had been warned not to “reach for it [the gun]."

Reynolds and her 4-year-old daughter were also in the car when the incident happened.