Harper's family, which held a rally Tuesday night outside a Riverdale Village Board meeting, says the shooting was unjustified.
"He never got out of the car. He sat there and this man shot him in the heart one time, killed my child for no reason," said Nora Harper, Brandon's mother.
Riverdale police say Harper and two others refused to comply with orders and the officer feared for his life.
The incident is under investigation by the state police and Cook County's public integrity task force.