The incident occurred on February 5 near the UIC-Halsted station on the Near West Side.
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Patrick Waldon, 31, was arrested the next day for allegedly shooting and robbing a 30-year-old man he sat next to on the train. Police said Waldon shot the victim in the back during a struggle over the victim's backpack.
Monday, police released a photo of another person of interest. They said the person they are looking for was with Waldon at the time of the shooting.
The shooting victim was transported to Stroger Hospital in serious condition at the time.
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Waldon has been charged with one count of aggravated battery/discharge of a firearm, one count of armed robbery, one count of armed habitual criminal and one issuance of a warrant.
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