The encounter started as a traffic stop and turned into a foot chase.
Prosecutors had argued before a jury that the first shot was reasonable, but the second shot was not, because Heaggan-Brown had no reason to fear for his life once Smith was on the ground, unarmed.
"Heaggan-Brown fired once at which time he observed the pistol fly out of Smith's hands and over the fence into the backyard of the residence", a criminal complaint states. The video showed the second shot - 1.69 seconds later - hit Smith in the chest as he lay on the ground.
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.
He was sacked from the police force, not because of the homicide charges he faced, but because of the allegations of sexual assault.
Attorneys for a former Milwaukee police officer acquitted in the fatal shooting of a black man say the officer always believed he was justified in using deadly force. "And our argument is that justification did not change over the course of 1.69 seconds between shots", defense attorney Jonathan Smith told the jury.
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Heaggan-Brown, 25, faced 60 years in prison. He was sacked after being accused of the sexual assault of another man.
Sylville Smith was killed in August 2016. Jurors are also being given the option to choose from two lesser charges instead second-degree reckless homicide and homicide by negligent operation of a risky weapon.
The defense team also "argued that Heaggan-Brown was following officer training in ending a threat, and that the former officer could not know whether Smith had another weapon", WUWM added. After Smith was shot, the resulting violence during protests left businesses burned, police cars damaged and officers injured.
Heaggan-Brown chose not to testify in the trial. Heaggan-Brown and two officers were making their way back to their district station when they chose to initiate one last traffic stop. "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".
And all six Baltimore officers charged over the 2015 death of Freddie Gray, due to spinal cord injuries suffered in the back of a police van, were eventually cleared.
While activist groups such as Black Lives Matter have drawn attention to police shootings in recent years, acquittals of police officers have become all but expected in these sorts of incidents.
The 12 person jury, which included four blacks, sided with the defense.