Slager shot and killed the 50-year-old Scott following a traffic stop for a busted taillight on April 4, 2015 while Slager was a member of the North Charleston Police Department.
An emotional Slager told the Scott family that he was grateful for their forgiveness.
Dr. Morgan wrote in a report after meeting with Slager that the former officer did not remember Scott being on top of him.
The judge's decision comes after Slager, who is white, pleaded guilty to a federal civil rights offense.
Wtness video surfaced that showed Slager shooting Scott eight times as he ran away. A judge sentenced him to 20 years in prison, according to The Associated Press, which had a reporter at the sentencing.
A white SC 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.
"He will never see me play high school football, never see me graduate", Scott's son Miles said in court on Wednesday.
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In addition, Fishman said Slager made no claims that Scott punched him, was on top of him or was stunned with the officer's taser by Scott.
Wilson, who prosecuted Slager in his state trial which ended in a hung jury, got into a heated exchange with Savage.
Norton's sentence later Thursday will bring to an end one of the most notorious criminal cases in modern S.C. history, a case that attracted worldwide attention after a bystander's video of Scott's fatal shooting went viral on the Internet.
After the sentencing, Scott's younger brother, Rodney Scott, said the family was thankful for the sentence.
A white former SC police officer has been sentenced to 20 years in prison for shooting an unarmed black motorist to death in 2015.
Slager faced murder charges in state court, but a jury in that case deadlocked a year ago, and the charges were dropped as part of his federal plea deal. Earlier Thursday, U.S. District Judge David Norton ruled that the killing was second-degree murder and said he'd work from guidelines that recommend Slager spends 19 to 24 years in prison.
Before Slager's sentence was handed down, the judge had to decide whether the shooting amounted to second-degree murder or manslaughter. Slager, who is white, fired five times into Scott's back as he ran away.
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"I forgive Michael Slager", mother Judy Scott said. Slager contended he was securing the weapon.
Slager, 36, pleaded guilty in federal court in May to violating Scott's civil rights by shooting Scott without justification.
A pre-sentencing report for Slager found that he committed manslaughter and recommended 10 to almost 13 years in prison.
Convictions in police officer shootings are uncommon in the US and prison time is even rarer.
Slager's attorney Andy Savage said Scott was partly responsible because he grabbed Slager's taser and tried to use it on the ex-cop.
SC has been aggressive in charging white officers who shoot unarmed black people.
During the trial, Slager testified that he was scared and felt "total fear that Mr. Scott was coming toward me". Groubert was sentenced to five years behind bars.
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