ST. LOUIS » Marcus Johnson Jr.’s parents figured a sunny day at a city 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.

"There was a line drawn," said funeral director Ronald Jones, referring to an unwritten code of the streets that protected the innocent from violence. "Kids, family — they were off-limits."

In an unrelated incident, a 1-year-old boy was shot in both legs Tuesday in another north St. Louis city park. The child survived the shooting.

Police say Marcus’ shooting remains under investigation but declined to discuss specifics. St. Louis police officer Don Re was less reticent in his personal blog, describing in detail his response to a "senseless death" when he was called to the hospital where another officer had driven the child in his patrol car rather than wait for an ambulance to arrive.

"Where the little boy’s heart had laid so close to the officer’s own heart was a mess that told us things would not end well," he wrote. "We were all hoping, but we also knew that it was going to take a miracle for that boy to live. He was not granted that miracle."

"Why did this boy have to die?" he continued. "Was it disrespect? Drugs? A woman? Money? All stupid reasons to fire a gun anywhere near another human being, let alone children, but here we are again, with another child lost to violence."

The boy’s parents told The Associated Press in an interview Wednesday that 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. "I saw the expression on his face and didn’t feel comfortable anymore."

Marcus Johnson Sr., 33, said his son was unrelentingly cheerful despite the heart condition, which required daily medication to control inflammation of his blood vessels and added vigilance to prevent falls that could lead to bruising and possible blood clots.

"He’d come in the room and smile," his father said through tears, the hospital visitor’s badge from two weeks ago still on his sweatshirt. "He’d light up the whole room."

Community support has been swift. After reports that the family needed $5,000 to cover funeral costs, an online fundraising campaign by a St. Louis alderman generated nearly three times that amount. Jones said the family will need that help and more to cover unpaid medical costs for their 15-year-old son, who was shot in the ankle, and to move out of a neighborhood where they have become a target since the shooting.