According to authorities, the shooting happened at about 1 a.m. Wednesday near Douglas Park.
Police say the officer was pulling out of an alley in the 3100-block of West 15th Place near Kedzie when he noticed a group of four people creating what was described as some sort of disturbance. The off-duty officer approached the group with his car, identified himself, and told them to keep the noise down, police said. One of the people -- whom police say was carrying a weapon -- allegedly approached the officer's car. That's when authorities say the officer fired, hitting Antonio Cross, 39, in the hand and his 22-year-old girlfriend in the head. The detective appears to have fired four or five shots.
The woman was said to be in very critical condition at Mt. Sinai Hospital. Cross was discharged from the hospital Wednesday.
Police have not recovered a weapon so far and it's not clear whether the woman was hit accidentally.
A relative of Cross who was at the scene at the time of the shooting says the group was all hanging out together at the park, and his cousin and his girlfriend did nothing wrong.
"Nobody had a gun," Leo Coleman said."It's at least 60 to 70 people sitting in the park, and nobody had a gun -- just everybody hanging out. That's it...no gun nowhere."
"That girl's going to die for no apparent reason," Coleman told ABC7 early Wednesday morning. "He got shot in the hand. Said the bullet went clear through, but they won't let us see him because he is under arrest. Under arrest for what?"
Police say Cross is still in custody. No charges have been filed against him.
Coleman says the woman is from Chicago's south suburbs.
Verna Pearson witnessed the shooting and had been taking to the female victim minutes earlier.
"They were arguing and she said, Miss Pearson can you give me bus money to help me get home, and I said, yes I can, 'cause I want you off these streets," said Pearson. "And the next thing I know I hear gunshots."
Dr. Gary Merlotti, who treats trauma patients, says an injury like the one the woman suffered can be extremely serious.
"The primary problem with gunshot wounds to brain is that they cause a lot of bleeding, a lot of blood that needs to be evacuated. The second issue is a lot of swelling. There are a lot of critical-care maneuvers we undertake," Merlotti said.
Other police officers who were already in the Douglas Park area say they heard the shots Wednesday morning and rushed to the scene. Two other people allegedly involved in the incident were apprehended.
Police markers could be seen later Wednesday where the bullet casings landed. A forensics team also evaluated the scene.
Chicago police say the off-duty officer announced he was a detective before he opened fire. The officer is known around the neighborhood.
Authorities say an independent investigation into the shooting is under way.