The shooting occurred at about 1:22 a.m. in the 11400-block of South Davol Avenue.
Responding officers found a group of people, including two men, 35 and 18 years old, who were involved in an altercation, police said.
Police said as officers tried to separate the men, an suspect's gun went off, striking a victim and an officer's protective vest.
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Another officer returned fire, striking the suspect and accidentally striking the victim, CPD officials said.
The 18-year-old victim was transported to the University of Chicago Medical Center in fair condition.
The officer hit in the vest was taken to a hospital and was later discharged.
The 35-year-old suspect, identified by police as Darren Brewster of Chicago, was taken to Christ Hospital in fair condition. Brewster faces a count of attempted murder and home invasion among other charges.
Brewster is expected in court Tuesday for a detention hearing.
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