Witnesses said the girl had just gotten off a ride in the Camp Cartoon Network area of the park Wednesday evening. She was running to rejoin her cousins and grandmother when she collapsed, Gurnee police said in a statement.
The girl's heart had stopped by the time paramedics arrived, and she was pronounced dead an hour later at a hospital, police said.
Police said the girl, whose name was not released, was from the Chicago suburb of Arlington Heights, about 20 miles south of Gurnee. An autopsy was planned to determine the cause of death.
"At this point we don't have any reason to believe this was ride-related," said Jim Taylor, a spokesman for Six Flags. "We will certainly participate and assist authorities in any way they ask us to."
Six Flags Inc. runs some 30 theme, water and zoological parks in North America.
In 2003, an 11-year-old girl from Gary, Indiana, collapsed at the park and died after riding the "Raging Bull" roller coaster. A coroner's jury later ruled that a heart condition caused Erica Emmons' death.