ST. LOUIS — Marcus Johnson Jr.’s parents figured a sunny day at a park was just what their son, a 6-year-old kindergartner, needed while recovering from heart surgery the previous week and a doctor’s visit that same day.

Instead, the family will bury Marcus on Thursday after the child was shot in the chest and killed in an attack his mother and father said stemmed from a traffic dispute. The boy’s 15-year-old brother and a 69-year-old family friend were also wounded by the occupants of a car who shot at the family’s minivan as it left O’Fallon Park on the city’s north side on March 11. Marcus Johnson Sr. said he returned fire in self-defense as the rolling shootout continued for several blocks, with three other children — ages 8, 10 and 11 — in the car.

Even as St. Louis experiences a crime surge that includes a 2014 homicide rate that was one of its highest in nearly two decades, the young heart patient’s death has rattled the city. The elder Johnson said Marcus, his youngest son, was diagnosed with a heart ailment as an infant, and he and his wife, Qiana Johnson, lost another child in 2012 who suffocated in his sleep at 4 months old.

Police say Marcus’ shooting remains under investigation but declined to discuss specifics.

The boy’s parents said the dispute began when Marcus Johnson Sr. had a brief conversation with an acquaintance in a nearby car that stopped while the two chatted. A passenger in another vehicle got out of that car to express his displeasure, they said.

“He just looked at us like he had the devil in his eyes,” said Qiana Johnson, 34, a stay-at-home mother who is five months pregnant.