Police said shortly before 7 p.m. their marine unit responded to a report that a person allegedly fell off a boat in the "Playpen" near the Jardine Water filtration plant.
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When officers arrived, they began looking for the missing person.
Police also said the Coast Guard brought an unconscious person who had been on a boat to shore. That person was taken to Northwestern Hospital in critical condition.
The marine unit searched the lake for the missing person, but the search has been suspended for the night.
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Chicago police said the two people were on the same boat. The Coast Guard said the missing person was wearing a red shirt and blue jean shorts.
No further information has been released.