Witnesses said they heard some half dozen shots being fired near South Byron Street and McClintock Avenue South around 12:20 p.m.
They rushed over to see a man lying on the ground, and called 911.
The victim, who is believed to be in his late teens or early 20s, sustained a gunshot wound to the chest. He was taken to an area hospital with life-threatening injuries.
Investigators believe the gunman is 15 or 16 years old. They believe he jumped into a car and fled the scene after the shooting. They are searching the area for an Asian or Hispanic teen wearing a blue jacket and a backpack.
Franklin High School and nearby John Muir Elementary School were placed under lockdown for several hours while detectives searched for the gunman. The lockdown was lifted before the students' dismissal time.
It was not known what prompted the shooting, and the investigation is ongoing.