The man got into an argument with another man shortly after 8:30 p.m. along 69th Street in Englewood, according to CPD. That's when the man pulled out a firearm and shot the victim, police said.
He suffered a gunshot wound to the thigh and was taken to University of Chicago Medical Center in grave condition.
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No one was in custody Monday morning, and Area One detectives are investigating.
CPD has planned to add new CTA train patrols and K-9 units amid a spike in crime on and around the transit system.