Chopper 7 was over the Des Plaines River at Oak Spring Road. There is a canoe-launching area near where crews were seen searching the water.
The Libertyville Fire Department said police received a call on Thursday with credible information that the missing person was in this area.
Fire officials said that call prompted a non-emergency "swift, specialized search."
Libertyville fire and Lake County Forest Preserve officials are leading the search effort.
Further information was not immediately available.