Questions remain about the incident Sunday morning.
One man is in critical condition at Northwestern Memorial Hospital, and another is in good condition.
Chicago police said two men were causing some sort of disturbance on a stopped train near the Roosevelt station, just after 10 p.m.
It's still unclear exactly what that disturbance was, but that's when a male suspect fired shots at the other two.
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A 25-year-old was struck multiple times, and rushed to Northwestern Hospital in critical condition, and a 20-year-old was taken to Stroger Hospital in good condition.
Police say a weapon was found on the scene.
CPD said a person of interest is being questioned, but it was not immediately clear if that is the suspect who allegedly fired those shots.
Detectives are still investigating.
Red Line service was affected for several hours overnight, but resumed as normal just before 2 a.m.
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