Police said it happened near the South Loop neighborhood's Roosevelt station, which offers Red Line, Green Line, and Orange Line service, just before 7 a.m. on May 15.
CPD released a photo of the suspect, who they say exposed himself, committed a lewd act, and made unwanted contact with a victim.
The suspect is described as 30-35 years old, about 6 feet tall, and 150-160 pounds with a scruffy and patchy beard.
CPD Mass Transit Detectives asked anyone with information to call them at 312-745-4447 or submit an anonymous tip at CPDTIP.com and use reference #JK255153.
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