Police responded Sunday to a 911 call at LaSalle Lake State Fish and Wildlife area, the Illinois Department of Natural Resources said Monday in a release.
The girl had been wearing a life jacket and was found onshore with park visitors. She had been on the 17-foot (5-meter) boat with her 53-year-old grandfather and her 80-year-old great-grandfather. The two men were not wearing life jackets, according to police.
They were pronounced dead by a coroner after their bodies were recovered about 7:45 p.m. Sunday.
The girl's grandfather was from Merrillville, Indiana, while her great-grandfather lived in Gary, Indiana.
Police were investigating how the boat capsized. The 2,000-acre (809-hectare) lake is near Marseilles, about 84 miles (135 kilometers) southwest of Chicago.
