Police say an off-duty 25th district officer saw the initial shooting and gave chase when he witnessed a teenager flee the scene.
At some point in that pursuit, the teenage victim was shot and killed. The victim's identity has not yet been released.
Police have not confirmed if the teen who was killed was the shooting suspect.
The victim of the initial shooting, an 18-year-old man, was later listed in critical but stable condition at a local hospital.
The incident is being investigated by internal affairs.
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