It's the first time the August 13, 2016, encounter between then-Milwaukee officer Dominique Heaggan-Brown and Smith, 23, was shown publicly. The shooting after a traffic stop sparked days of unrest in Milwaukee, a city long torn by racial tensions.
Heaggan-Brown, who was later fired from the department in an unrelated sexual assault investigation, was charged in December with first-degree reckless homicide.
Body camera footage from a second officer showed that Heaggan-Brown fired a second, and fatal, shot after Smith hurled his weapon over a fence and that Smith had his hands near his head, an earlier complaint said. According to the complaint, 1.69 seconds separated the two shots.
Prosecutors say Heaggan-Brown's first shot was justified, but not the second, according to CNN affiliate WISN.
The reaction in court, including sobs from Smith's family, to the video caused the judge to clear the courtroom on the third day of Heaggan-Brown's trial, which included testimony from his partner at the time of the shooting, the affiliate reported.
Heaggan-Brown's defense attorney called for a mistrial, saying the family's reaction could influence the jury, according to CNN affiliate WITI.
"At least one young man doubled over and at some point stood up and left," Heaggan-Brown's attorney Steven Kohn said in court, WITI reported. "That is an editorial comment that is injected into these proceedings."
Judge Jeffrey A. Conen denied the request.
"The court does not believe that this has arisen to the level, at this point, of a mistrial," Conen said, CNN affiliate WTMJ reported.
In court, Heaggan-Brown's former partner, Ndiva Malafa, testified that they were chasing Smith because they saw he had a gun.
"I saw Mr. Smith exit the vehicle. I observed the firearm and at that point, we made eye contact. At that moment, I believe I started to -- I see him running northeast. Out of the corner of my eye, I see Heaggan-Brown chase him as well,"" Malafa testified, WITI reported.
Malafa's body camera footage was played several times in court, according to WTMJ. Malafa also guided the jury through the footage frame by frame, the station reported.
The video picks up as Malafa jumps out of this squad car. The shaky footage shows him trailing behind Heaggan-Brown, who is chasing Smith. Smith runs across lawn, turns the corner and heads toward a fence. He slips right before he reaches the fence.
According to a complaint, Smith was armed with a Glock .40-caliber model 22 semi-automatic handgun with an extended magazine. Heaggan-Brown''s body camera showed he threw the weapon in an adjacent yard as he was shot the first time, the complaint said.
The circumstances surrounding Heaggan-Brown's second shot will be key in the trial.
An autopsy showed that Smith suffered a gunshot wound through his upper right arm and another to his right upper chest.
Police have said Heaggan-Brown shot Smith after he failed to comply with orders to put down his gun, which was loaded with 23 rounds.
In the body camera audio, which was activated 30 seconds after the shooting, Heaggan-Brown was heard yelling at Smith: "Stop reaching." The ex-cop moved Smith's hand away from his waist, the complaint said.
Heaggan-Brown had previously said he believed Smith "was reaching for his waist so he discharged his weapon a second time."
In an interview with WITI, Smith's brother Sedan said: "It's the longest 30 seconds of my life to be able to just watch the video."
Kohn, in an interview with CNN, said the video caused "a very dramatic and long lasting reaction with people making noise, walking out, sobbing and drawing my attention and the attention of the Judge away from the evidence being presented.
"That is something that can't be abiding," he added.
Heaggan-Brown faces a maximum prison sentence of 60 years if convicted.
The prosecution wrapped up Wednesday's proceedings by finishing their questioning of Malafa. The trial is expected to continue into late next week.