What the McHenry Township Fire Protection District is calling a boating accident happened in the 2400-block of Riverside Drive West.
Illinois state Sen. Craig Wilcox said at least one person died from their injuries, and passengers on what he described as the "offending boat" may be on foot in the area of Holiday Hills.
Wilcox asked residents to call police if they notice anything suspicious.
"Our thoughts and prayers for everyone involved in the boating accident just down river from the dam. We understand there has been a victim who has succumbed to injuries, and that members of the offending boat may be on foot in Holiday Hills area south of the dam. Please keep your eyes open for anyone who may be injured and appear to be a boater that is out of place. Call the police even if you have just suspicions," Wilcox said in a statement.
The McHenry County Sheriff's Office said the river was closed south of the McHenry Dam and asked residents to avoid the area as authorities investigated. The waterway has since reopened.
Officials did not immediately provide further information.
This is a developing story. Check back with ABC7 for updates.