When they crossed paths last weekend in a lethal confrontation that started with a traffic stop, Smith's relatives and friends said it wasn't their first encounter.
A pair of officers had stopped Smith and another man in a car that afternoon when the men bolted on foot. Smith was black, as is Heaggan.
Smith's sister, Sherelle, said her brother and the officer attended the same school at one time.
"The officer knew him personally from high school and he still shot him," Sherelle Smith told CNN on Wednesday. "He didn't like my brother. The officer had a career, but my brother was more popular. He used to harass Sylville."
Milwaukee public school district spokesperson Katie Cunningham told CNN that Smith and the officer did not attend high school together, according to their records.
On Thursday, CNN asked Sherelle Smith about the school district's assertion that there is no record of the two having attended the same school.
Smith said the school district is wrong: "They knew each other from school and the neighborhood."
Police have not named the officer.
CNN has sought comment from Heaggan with no response. Officers have been stationed outside his home and local media reported that he has been staying elsewhere.
The Milwaukee police union has not responded to requests for comment.
Sgt. Timothy GauerkeI, a Milwaukee police spokesman, said he was familiar with reports that the men attended the same school and knew each other but could not confirm them.
Heaggan's name and photo have been widely circulated on social media, with angry comments from people who have threatened to have him killed.
Some online posters described encounters with what they said was an overzealous officer.
A Sherman Park community activist, Goddess Mathews, 30, said it was known in the neighborhood that the officer and Smith were onetime classmates.
"No one doubts the officer knew Sylville. How could he not?" Matthews said.
Mathews remembered Smith as a popular dancer and the officer as an "aggressive" cop who was not well liked in the community.
"Yes, a black man did the shooting but he was wearing a blue uniform with a badge," she said.
"He represented the mentality that people around here are less than human. We're enemies of the state."
Mathews said many Milwaukee police officers treat residents of the predominantly black Sherman Park neighborhood with dignity and respect but Heaggan wasn't one of them.
"He was like a tyrant," she said. "He took a very aggressive approach on the street."
Mathews said the officer was a familiar figure in the hip-hop nightclub scene.
"He could be overly aggressive out here but, in a room with 200 of us and without his badge and uniform, he wasn't so tough," she said.
Sherelle Smith said Heaggan also knew her brother from hanging out at a downtown nightclub popular for Hip Hop and R&B.
"He would see my brother at the clubs," she said.
Deandre Ueal, 25, said he knew Smith from the Boys & Girls Club and the officer from the local basketball courts when they were younger.
Ueal said the officer had a reputation "for messing with people for no reason" and always seemed "eager to bust people up."
As for Smith, Ueal said, "He grew up rough. We all grew up in poverty. It was hard."
The cop has six years of service with the Milwaukee police -- three as an officer, officials said. He entered the department as part of an apprentice program that recruits high school students and requires that they complete college credits.
He was assigned to District 7 on the northwest side.
The officer has been placed on administrative duty during an investigation.
Darius, a young man who asked to be identified only by his first name, said Heaggan "loves being a cop."
Heaggan wouldn't speak about life on the force but preferred to talk instead about "being positive," Darius said.
Darius remembered being with Heaggan at a store when some people tried to start a fight with them. The officer told him, "Let's get out of here. We don't want any trouble."
Asked about Smith's shooting and the police claim that he refused to put down his weapon, Darius said: "I feel if you're doing something and cops catch up with you, you got to give it up."
Cedric Jackson, who retired in 2011 after more than two decades as a Milwaukee police officer, told CNN that he didn't know the cop involved in the Smith shooting. But Jackson said younger officers are taught to show people in the community "who's boss and keep them in fear of you."
There also are frequent attempts to humiliate people in impoverished sections of the city, Jackson said.
"During questioning or stops, officers would often ask black men, 'Are you working?'" he recalled. "When they said, 'No,' the officer would say, 'What a shame. How can you consider yourself a man and not work? If I had children, I'd shovel s--- if I had to.'"
After Saturday's shooting, Mayor Tom Barrett said police body camera footage showed Smith holding a handgun during the encounter. That video has not been released.
Police said the officer shot Smith after he failed to comply with orders to put his gun down, which was loaded with 23 rounds -- more than the officer was carrying.
Smith's sister, Kimberly Neal, mourned a brother she described as a high school graduate who played basketball, not a "man with a lengthy arrest record" described by police.
On Thursday, the Wisconsin Department of Justice announced an anonymous tip line for anyone with information related to the shooting.
"It is my goal to complete officer-involved death investigations expeditiously so the community gets the answers they deserve," said Attorney General Brad Schimel in a statement.
The findings of its investigation will be turned over to the Milwaukee County district attorney's office.
Protests over the shooting turned violent during the weekend as at least six businesses were torched, cars were burned and four officers were injured in the clashes.
The damage was not as extensive Sunday night, but shots rang out in three locations, and an 18-year-old man who was struck in the neck was hospitalized, police said. At least seven officers were wounded, including some who were hit by rocks and bricks.
The area has been calmer in recent days but the shooting brought festering racial tension to the surface. Community leaders said the violence was a byproduct of inequities, injustice, unemployment and lack of educational opportunities.
On virtually every indicator of racial inequality, Milwaukee consistently ranks among the worst five cities
in the country.