No police will be charged in the shooting of African American Jacob Blake, who was left paralyzed in an incident which sparked unrest in the US city of Kenosha in August, the prosecutor announced Tuesday.

Kenosha County District Attorney Michael Graveley said that white police officer Rusten Sheskey had a reasonable claim of fear of being stabbed when he shot Blake several times in the back as Blake tried to get in his car.

Graveley said it was a "very narrow decision," but that based on existing law for officer-involved shootings, it would be difficult for state prosecutors to prove that Sheskey was not engaged in self-defense if he or others were charged in the case.

"No Kenosha law enforcement officer in this case will be charged with any criminal offense, based on the facts of the law," said Graveley.

The shooting of the 29-year-old on August 23 in the Wisconsin town poured fuel onto nationwide anger over police shootings of Black Americans, sparking several days of violent protests.

Bystander video showed Sheskey firing several shots into Blake's back as he tried to get into his car while his three children sat inside.

Blake survived but was left paralyzed, without the use of his legs.

Graveley said officers had reason to believe Blake was dangerous based on 911 calls to the scene from his estranged fiancee, an arrest warrant out for Blake on domestic abuse charges, and what Blake said to officers as he sought to leave the scene with his three children in the car.

The district attorney also said that officers made three attempts to taze Blake to subdue him, which all failed.

Graveley added it was "absolutely incontrovertible" that Blake had armed himself with a knife while being confronted by the police, and that Blake himself had admitted as much.

He added that Blake will also not be charged with any crime in the incident.

The shooting sparked three nights of violent protests which culminated on the night of August 25 when, drawn to the city by calls from right-wing militia, a 17-year-old carrying an assault rifle, Kyle Rittenhouse, shot dead two protestors and wounded a third.

Earlier Tuesday Rittenhouse entered a plea of "not guilty" to murder charges in the shootings.

Ahead of Graveley's announcement the city girded for a new burst of protests.

Kenosha businesses boarded up shopfronts in preparation for possible violence and Wisconsin Governor Tony Evers ordered the state's National Guard to mobilize 500 guardsmen to help Kenosha law enforcement if needed.

Blake's case fed into the election battle between Democrat Joe Biden and President Donald Trump, with Biden offering support for Blake's family and decrying systemic racism in law enforcement, while Trump expressed support for the police plus law and order, and for Rittenhouse.

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