A white former South Carolina officer was sentenced to 20 years in prison on Thursday for fatally shooting an unarmed black motorist in the back in 2015, wrapping up a case that became a rallying cry for the Black Lives Matter movement.

Attorneys for ex-North Charleston Officer Michael Slager said he shot 50-year-old Walter Scott in self-defense after the two fought and Scott grabbed Slager's stun gun. They said race didn't play a role in the shooting and Slager never had any 'racial animus' toward minorities.

Still, Slager pleaded guilty in federal court to violating Scott's civil rights. As part of the plea agreement reached in May, prosecutors dropped state murder charges.  A year ago, a state judge declared a mistrial when jurors deadlocked in that case.

 'This is a tragedy that shouldn't have happened,' U.S. District Judge David Norton said.