The encounter started as a traffic stop and turned into a foot chase. Prosecutors contended Smith didn't pose a threat before Heaggan-Brown fired again. Smith was on the ground when he received the fatal shot.
Smith's death brought to the surface long-simmering tensions between black Milwaukee residents and police, and demonstrators assembled near the site of the shooting in Sherman Park hours after it happened.
Heaggan-Brown was sacked from the Milwaukee Police Department last fall on sexual assault charges, according to the New York Times. Jonathan Smith, Officer Heaggan-Brown's lawyer, told jurors that officers were taught to use the "one-plus rule" - or to expect that if a person has one weapon, he might have another.
Heaggan-Brown, 25, faced 60 years in prison. The sexual assault case wasn't mentioned during the trial because it's being tried separately.
FILE - This undated file photo provided by the Milwaukee County Sheriff's Office shows Sylville Smith. Not only did they acquit him on the first-degree reckless homicide charge, but they also declined to choose lesser options that included second-degree reckless homicide and homicide by negligent operation of a risky weapon.
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After two days of deliberations, jurors found ex-officer Dominique Heaggan-Brown not guilty of first-degree reckless homicide in the death of Smith. Heaggan-Brown was acquitted of all charges Wednesday.
News of Dominique Heaggan-Brown's acquittal has sparked plenty of reactions on Twitter. He was charged with assaulting a man a day after he fatally shot Smith.
In a request filled with tears, the sister of an unarmed black man killed by a former Milwaukee police officer called for the community to come together and be peaceful after a not-guilty verdict. "It's not. We have to go back - and I can't tell exactly how many frames but we have to go back two-tenths or three-tenths of a second - we have to go back several frames ... to delve into the decision-making process that goes into firing this shot".
Smith - who had a "lengthy arrest record" - was lying on the ground when he received another shot that killed him. As they exited their squad auto, Mr. Smith, who was armed with a handgun, darted away and ran into a yard with a chain-link fence.
The 12-member jury included four African-Americans. The following morning Heaggan-Brown reportedly left the man at a Milwaukee area hospital, then text a Police Department superior to say he had made a mistake and wanted "to handle this in the most secret and right way possible".