A "person of interest" was taken into custody after the shooting at Don Carter Lanes on East State Street, Rockford Police Chief Dan O'Shea said during a news conference.
WATCH: Rockford Police Chief Dan O'Shea give latest on shooting
6 shot, 3 killed in apparent random shooting at Don Carter Lanes bowling alley in Rockford, IL
O'Shea said he did not think any officers fired their weapons, but the shooter was active when police arrived on the scene.
Police said a 37-year-old man was in custody in connection with the incident.
Two teenagers were among those shot, O'Shea said, but he did not know their ages.
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Rockford police asked the public to avoid the area during the following investigation in a tweet about 7:15 p.m.
O'Shea said Rockford police received calls for shots being fired at the bowling alley at 6:55 p.m.
Republican U.S. Rep. Adam Kinzinger, whose district includes Rockford, tweeted he was "Praying for the victims and their families tonight after the heinous attack in our community."
Rockford is about 80 miles northwest of Chicago.
ABC News and Associated Press contributed to this report.