Police said one man and two teenage girls were killed in the shooting. The fourth person was taken to the hospital. Her condition was unknown.
"We have three deceased individuals inside the apartment and one adult male and two teenage girls," said St. Charles Police Deputy Chief David Kintz.
Police said officers were called to the condo in the 400 block of South 1st Street around 5 p.m., after neighbors heard gunshots.
"When I was walking across the street, I heard screaming and I thought it was just kids screaming, and I looked up and there was like a door open and I thought that was weird. It is wintertime. It is really cold," said Tammy Hartje, who lives across the street.
Investigators said they were not looking for any suspects.
"The preliminary investigation at this time shows this is a domestic-related incident and we do not believe there is any danger to the public at large right now," said Kintz.
Police were still trying to determine a motive.
"It is tragic. It doesn't have to happen. Never has to happen like that," said Larry Kling, who lives across the street.
Police say they also received one 911 call from inside the home.
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